Add the base class for decoder buffers used in cast media.

chromecast/media/media.gyp

Index: chromecast/media/media.gyp
diff --git a/chromecast/media/media.gyp b/chromecast/media/media.gyp
index c6f70301748ab048cd8c84587fc50d2951d2f3c3..bac79dad4eb121a6b2f8912d07160d43c4a3e65e 100644
--- a/chromecast/media/media.gyp
+++ b/chromecast/media/media.gyp
@@ -20,6 +20,10 @@
         'cma/base/buffering_state.cc',
         'cma/base/buffering_state.h',
         'cma/base/cma_logging.h',
+        'cma/base/decoder_buffer_base.cc',
+        'cma/base/decoder_buffer_base.h',
+        'cma/base/decoder_buffer_wrapper.cc',
+        'cma/base/decoder_buffer_wrapper.h',
       ],
     },
     {