Allow media track switching.

content/renderer/media/media_interface_provider.h

Index: content/renderer/media/media_interface_provider.h
diff --git a/content/renderer/media/media_interface_provider.h b/content/renderer/media/media_interface_provider.h
index d761fdf705214f0e11c4a6a778d3b17d1d0b2625..5194dd455657c7be85f38902d4ebc9133b0b80af 100644
--- a/content/renderer/media/media_interface_provider.h
+++ b/content/renderer/media/media_interface_provider.h
@@ -7,6 +7,7 @@
 
 #include "base/callback.h"
 #include "base/macros.h"
+#include "base/single_thread_task_runner.h"
 #include "base/threading/thread_checker.h"
 #include "content/common/content_export.h"
 #include "media/mojo/interfaces/interface_factory.mojom.h"
@@ -21,7 +22,8 @@ namespace content {
 
 // MediaInterfaceProvider is an implementation of mojo InterfaceProvider that
 // provides media related services and handles disconnection automatically.
-// This class is single threaded.
+// This class will forward GetInterface calls to the creating thread (i.e. the
+// thread where constructor was called) if necessary.

[xhwang, 2017/03/29 00:16:47]

That's impl detail. You can probably just say "GetInterface() can be called on
any thread".

[servolk, 2017/03/29 01:49:33]

Done.

 class CONTENT_EXPORT MediaInterfaceProvider
     : public service_manager::mojom::InterfaceProvider {
  public:
@@ -41,6 +43,10 @@ class CONTENT_EXPORT MediaInterfaceProvider
   service_manager::InterfaceProvider* remote_interfaces_;
   media::mojom::InterfaceFactoryPtr media_interface_factory_;
 
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+  base::WeakPtr<MediaInterfaceProvider> weak_this_;
+  base::WeakPtrFactory<MediaInterfaceProvider> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(MediaInterfaceProvider);
 };