Don't re-randomize picture_id/tl0_pic_idx when re-initializing internal encoders.

TESTED=video_loopback and AppRTCMobile with forced encoder reinits every 30 frames.
BUG=webrtc:7475

Review-Url: https://codereview.webrtc.org/2833493003
Cr-Commit-Position: refs/heads/master@{#17984}

webrtc/modules/video_coding/utility/simulcast_rate_allocator_unittest.cc

@@ -36,6 +36,7 @@ class MockTemporalLayers : public TemporalLayers {
                void(bool, CodecSpecificInfoVP8*, uint32_t));
   MOCK_METHOD2(FrameEncoded, void(unsigned int, int));
   MOCK_CONST_METHOD0(CurrentLayerId, int());
+  MOCK_CONST_METHOD0(Tl0PicIdx, uint8_t());
 };
 }  // namespace