New helper function test::ReadI420Buffer, refactor FrameReader to use it.

webrtc/modules/video_coding/codecs/test/videoprocessor_unittest.cc

Index: webrtc/modules/video_coding/codecs/test/videoprocessor_unittest.cc
diff --git a/webrtc/modules/video_coding/codecs/test/videoprocessor_unittest.cc b/webrtc/modules/video_coding/codecs/test/videoprocessor_unittest.cc
index 148d8dc74ab6b2629420e40568324604b24950a8..dcfa52d7283bec9e1e27c48dfc8e147ced348389 100644
--- a/webrtc/modules/video_coding/codecs/test/videoprocessor_unittest.cc
+++ b/webrtc/modules/video_coding/codecs/test/videoprocessor_unittest.cc
@@ -75,7 +75,7 @@ TEST_F(VideoProcessorTest, Init) {
 TEST_F(VideoProcessorTest, ProcessFrame) {
   ExpectInit();
   EXPECT_CALL(encoder_mock_, Encode(_, _, _)).Times(1);
-  EXPECT_CALL(frame_reader_mock_, ReadFrame(_)).WillOnce(Return(true));
+  EXPECT_CALL(frame_reader_mock_, ReadFrame()).WillOnce(Return(nullptr));
   // Since we don't return any callback from the mock, the decoder will not
   // be more than initialized...
   VideoProcessorImpl video_processor(