Chromecast: mitigate threading issue between AvPipeline/MediaKeys.

chromecast/media/cma/pipeline/av_pipeline_impl.h

Index: chromecast/media/cma/pipeline/av_pipeline_impl.h
diff --git a/chromecast/media/cma/pipeline/av_pipeline_impl.h b/chromecast/media/cma/pipeline/av_pipeline_impl.h
index 20b3fc4feda2e62fa917b28a5c3d7b0e4c32d2aa..60d80f03507bb2a167c2521bdeeb114b0e355684 100644
--- a/chromecast/media/cma/pipeline/av_pipeline_impl.h
+++ b/chromecast/media/cma/pipeline/av_pipeline_impl.h
@@ -11,6 +11,7 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
+#include "base/synchronization/lock.h"
 #include "base/threading/thread_checker.h"
 #include "chromecast/media/cma/backend/media_component_device.h"
 #include "chromecast/media/cma/pipeline/av_pipeline_client.h"
@@ -157,6 +158,7 @@ class AvPipelineImpl {
   bool pending_time_update_task_;
 
   // Decryption keys, if available.
+  base::Lock media_keys_lock_;
   BrowserCdmCast* media_keys_;
   int media_keys_callback_id_;