[Cast] Fix referenced_frame_ids emitted by Vp8Encoder, with massive test clean-up.

The referenced_frame_ids set by Vp8Encoder were wrong in two ways:
1) A recent change introduced an off-by-one error.  2) Math was being
done with results stored in a uint8 variable, which was causing the
upper 24 bits to be stripped.

Added VideoEncoderImplTest.GeneratesKeyFrameThenOnlyDeltaFrames test to
prevent future regressions.  In addition, a lot of unit test code clean-up
was done to beef-up existing testing while also removing useless tests.

Review URL: https://codereview.chromium.org/581903002

Cr-Commit-Position: refs/heads/master@{#295493}

media/cast/receiver/video_decoder_unittest.cc

@@ -12,6 +12,7 @@
 #include "media/cast/cast_config.h"
 #include "media/cast/receiver/video_decoder.h"
 #include "media/cast/sender/vp8_encoder.h"
+#include "media/cast/test/utility/default_config.h"
 #include "media/cast/test/utility/standalone_cast_environment.h"
 #include "media/cast/test/utility/video_utility.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -26,7 +27,7 @@ const int kHeight = 240;
 const int kFrameRate = 10;
 
 VideoSenderConfig GetVideoSenderConfigForTest() {
-  VideoSenderConfig config;
+  VideoSenderConfig config = GetDefaultVideoSenderConfig();
   config.width = kWidth;
   config.height = kHeight;
   config.max_frame_rate = kFrameRate;
@@ -83,7 +84,12 @@ class VideoDecoderTest : public ::testing::TestWithParam<Codec> {
     // Test only supports VP8, currently.
     CHECK_EQ(CODEC_VIDEO_VP8, GetParam());
     vp8_encoder_.Encode(video_frame, encoded_frame.get());
+    // Rewrite frame IDs for testing purposes.
     encoded_frame->frame_id = last_frame_id_ + 1 + num_dropped_frames;
+    if (last_frame_id_ == 0)
+      encoded_frame->referenced_frame_id = encoded_frame->frame_id;
+    else
+      encoded_frame->referenced_frame_id = encoded_frame->frame_id - 1;
     last_frame_id_ = encoded_frame->frame_id;
 
     {

media/cast/sender/vp8_encoder.cc

@@ -134,7 +134,7 @@ bool Vp8Encoder::Encode(const scoped_refptr<media::VideoFrame>& video_frame,
   raw_image_->stride[VPX_PLANE_U] = video_frame->stride(VideoFrame::kUPlane);
   raw_image_->stride[VPX_PLANE_V] = video_frame->stride(VideoFrame::kVPlane);
 
-  uint8 latest_frame_id_to_reference;
+  uint32 latest_frame_id_to_reference;
   Vp8Buffers buffer_to_update;
   vpx_codec_flags_t flags = 0;
   if (key_frame_requested_) {
@@ -228,7 +228,7 @@ bool Vp8Encoder::Encode(const scoped_refptr<media::VideoFrame>& video_frame,
 
 uint32 Vp8Encoder::GetCodecReferenceFlags(vpx_codec_flags_t* flags) {
   if (!use_multiple_video_buffers_)
-    return last_encoded_frame_id_ + 1;
+    return last_encoded_frame_id_;
 
   const uint32 kMagicFrameOffset = 512;
   // We set latest_frame_to_reference to an old frame so that