Reland "Keep spatial_idx=kNoSpatialIdx(255) if there is no layer indices."

This is a reland of 47836b4ebb8a5c71695b5ec07bffd5ee4e3bc2ff

Internal tests are synced with the fix.

Original change's description:
> Keep spatial_idx=kNoSpatialIdx(255) if there is no layer indices.
> 
> spatial_idx is not present in RTP header if there is no temporal or
> spatial layering. But the parser sets spatial_idx to 0 in this case.
> When reflector repacketizes such packets it writes layering indices
> into outgoing packets. When packets arrive to receiver it thinks that
> it deals with multi layer stream and passes it through special path
> in Vp9 reference frame finder which never outputs inter frames.
> 
> I modified the parser such that it keeps spatial_idx=kNoSpatialIdx(255)
> when there is no layer indices in RTP header. Related unit tests have
> been modified as well.
> 
> Bug: none
> Change-Id: I14498cafb4e57797577dc873298c35b243479f88
> Reviewed-on: https://webrtc-review.googlesource.com/17980
> Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
> Reviewed-by: Erik Språng <sprang@webrtc.org>
> Reviewed-by: Åsa Persson <asapersson@webrtc.org>
> Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20560}

TBR=brandtr@webrtc.org,asapersson@webrtc.org,sprang@webrtc.org

Bug: none
Change-Id: I6087a8b20a926296b30432d69251670120b2a20c
Reviewed-on: https://webrtc-review.googlesource.com/20940
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20591}

modules/rtp_rtcp/source/rtp_format_vp9.cc

@@ -721,7 +721,6 @@ bool RtpDepacketizerVp9::Parse(ParsedPayload* parsed_payload,
   vp9->beginning_of_frame = b_bit ? true : false;
   vp9->end_of_frame = e_bit ? true : false;
   vp9->ss_data_available = v_bit ? true : false;
-  vp9->spatial_idx = 0;
 
   // Parse fields that are present.
   if (i_bit && !ParsePictureId(&parser, vp9)) {

modules/rtp_rtcp/source/rtp_format_vp9_unittest.cc

@@ -30,8 +30,7 @@ void VerifyHeader(const RTPVideoHeaderVP9& expected,
   EXPECT_EQ(expected.picture_id, actual.picture_id);
   EXPECT_EQ(expected.max_picture_id, actual.max_picture_id);
   EXPECT_EQ(expected.temporal_idx, actual.temporal_idx);
-  EXPECT_EQ(expected.spatial_idx == kNoSpatialIdx ? 0 : expected.spatial_idx,
+  EXPECT_EQ(expected.spatial_idx, actual.spatial_idx);
-      actual.spatial_idx);
   EXPECT_EQ(expected.gof_idx, actual.gof_idx);
   EXPECT_EQ(expected.tl0_pic_idx, actual.tl0_pic_idx);
   EXPECT_EQ(expected.temporal_up_switch, actual.temporal_up_switch);

video/video_send_stream_tests.cc

@@ -3114,8 +3114,23 @@ class Vp9HeaderObserver : public test::SendTest {
     EXPECT_EQ(kMaxTwoBytePictureId, vp9.max_picture_id);       // M:1
     EXPECT_NE(kNoPictureId, vp9.picture_id);                   // I:1
     EXPECT_EQ(vp9_settings_.flexibleMode, vp9.flexible_mode);  // F
-    EXPECT_GE(vp9.spatial_idx, 0);                             // S
+
+    if (vp9_settings_.numberOfSpatialLayers > 1) {
       EXPECT_LT(vp9.spatial_idx, vp9_settings_.numberOfSpatialLayers);
+    } else if (vp9_settings_.numberOfTemporalLayers > 1) {
+      EXPECT_EQ(vp9.spatial_idx, 0);
+    } else {
+      EXPECT_EQ(vp9.spatial_idx, kNoSpatialIdx);
+    }
+
+    if (vp9_settings_.numberOfTemporalLayers > 1) {
+      EXPECT_LT(vp9.temporal_idx, vp9_settings_.numberOfTemporalLayers);
+    } else if (vp9_settings_.numberOfSpatialLayers > 1) {
+      EXPECT_EQ(vp9.temporal_idx, 0);
+    } else {
+      EXPECT_EQ(vp9.temporal_idx, kNoTemporalIdx);
+    }
+
     if (vp9.ss_data_available)  // V
       VerifySsData(vp9);
 
@@ -3257,15 +3272,26 @@ void VideoSendStreamTest::TestVp9NonFlexMode(uint8_t num_temporal_layers,
     }
 
     void InspectHeader(const RTPVideoHeaderVP9& vp9) override {
-      bool ss_data_expected = !vp9.inter_pic_predicted &&
+      bool ss_data_expected =
-                              vp9.beginning_of_frame && vp9.spatial_idx == 0;
+          !vp9.inter_pic_predicted && vp9.beginning_of_frame &&
+          (vp9.spatial_idx == 0 || vp9.spatial_idx == kNoSpatialIdx);
       EXPECT_EQ(ss_data_expected, vp9.ss_data_available);
-      EXPECT_EQ(vp9.spatial_idx > 0, vp9.inter_layer_predicted);  // D
+      if (num_spatial_layers_ > 1) {
+        EXPECT_EQ(vp9.spatial_idx > 0, vp9.inter_layer_predicted);
+      } else {
+        EXPECT_FALSE(vp9.inter_layer_predicted);
+      }
+
       EXPECT_EQ(!vp9.inter_pic_predicted,
                 frames_sent_ % kKeyFrameInterval == 0);
 
       if (IsNewPictureId(vp9)) {
+        if (num_temporal_layers_ == 1 && num_spatial_layers_ == 1) {
+          EXPECT_EQ(kNoSpatialIdx, vp9.spatial_idx);
+        } else {
           EXPECT_EQ(0, vp9.spatial_idx);
+        }
+        if (num_spatial_layers_ > 1)
           EXPECT_EQ(num_spatial_layers_ - 1, last_vp9_.spatial_idx);
       }