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

webrtc/test/testsupport/mock/mock_frame_reader.h

Index: webrtc/test/testsupport/mock/mock_frame_reader.h
diff --git a/webrtc/test/testsupport/mock/mock_frame_reader.h b/webrtc/test/testsupport/mock/mock_frame_reader.h
index f06750ef4a41cc23c1f8ce234c38efe8462f90a1..c609413ae90d9b6e0b1c5b389206b523f701466b 100644
--- a/webrtc/test/testsupport/mock/mock_frame_reader.h
+++ b/webrtc/test/testsupport/mock/mock_frame_reader.h
@@ -21,7 +21,7 @@ namespace test {
 class MockFrameReader : public FrameReader {
  public:
   MOCK_METHOD0(Init, bool());
-  MOCK_METHOD1(ReadFrame, bool(uint8_t* source_buffer));
+  MOCK_METHOD0(ReadFrame, rtc::scoped_refptr<I420Buffer>());
   MOCK_METHOD0(Close, void());
   MOCK_METHOD0(FrameLength, size_t());
   MOCK_METHOD0(NumberOfFrames, int());