Calculate VP9 references to wrap at kPicIdLength instead of 16 bits.

Bug: webrtc:8293
Change-Id: Iedc09a10548c2112e99247a5845a02c1bd3e7b80
Reviewed-on: https://webrtc-review.googlesource.com/4200
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20017}

modules/video_coding/rtp_frame_reference_finder_unittest.cc

@@ -1313,5 +1313,12 @@ TEST_F(TestRtpFrameReferenceFinder, Vp9FlexibleModeTwoSpatialLayersReordered) {
   CheckReferencesVp9(8, 1, 7);
 }
 
+TEST_F(TestRtpFrameReferenceFinder, WrappingFlexReference) {
+  InsertVp9Flex(0, 0, false, 0, 0, 0, 0, false, {1});
+
+  ASSERT_EQ(1UL, frames_from_callback_.size());
+  CheckReferencesVp9(1, 0, 0);
+}
+
 }  // namespace video_coding
 }  // namespace webrtc