Add a general PostProcessorUnittest.

chromecast/public/media/audio_post_processor_shlib.h

Index: chromecast/public/media/audio_post_processor_shlib.h
diff --git a/chromecast/public/media/audio_post_processor_shlib.h b/chromecast/public/media/audio_post_processor_shlib.h
index 35c368e4080fcbbc9d0bc864fd646a75c5238906..41a610ed7e88e091c0e7c8abf303a9b9eeed58d9 100644
--- a/chromecast/public/media/audio_post_processor_shlib.h
+++ b/chromecast/public/media/audio_post_processor_shlib.h
@@ -32,6 +32,9 @@ AudioPostProcessorShlib_Create(const std::string& config, int channels);
 namespace chromecast {
 namespace media {
 
+// The maximum amount of data that will ever be processed in one call.
+const int kMaxAudioWriteTimeMilliseconds = 20;
+
 // Interface for AudioPostProcessors used for applying DSP in StreamMixerAlsa.
 class AudioPostProcessor {
  public: