[Chromoting] Add a unit test to verify that the VP8 codec doesn't change color values too much.

remoting/base/codec_test.h

Index: remoting/base/codec_test.h
diff --git a/remoting/base/codec_test.h b/remoting/base/codec_test.h
index 40f02e2c29ce654a9b3b46d1ddde9044dcad4922..84217a491118cdc61e8d74255803d0f178608173 100644
--- a/remoting/base/codec_test.h
+++ b/remoting/base/codec_test.h
@@ -32,6 +32,12 @@ void TestEncoder(Encoder* encoder, bool strict);
 // are correct.
 void TestEncoderDecoder(Encoder* encoder, Decoder* decoder, bool strict);
 
+// Generate a frame containing a gradient, and test the encoder and decoder
+// pair.
+void TestEncoderDecoderGradient(Encoder* encoder, Decoder* decoder,
+                                double max_error_limit,
+                                double mean_error_limit);
+
 }  // namespace remoting
 
 #endif  // REMOTING_BASE_CODEC_TEST_H_