Add mailbox-based Mojo VideoFrame variant.

media/base/video_frame.h

Index: media/base/video_frame.h
diff --git a/media/base/video_frame.h b/media/base/video_frame.h
index 7ed1162b7a4147956f0d01d95c971ce7bbf4a0ab..3139f7a04326a883de75fbf3942454f9471289ae 100644
--- a/media/base/video_frame.h
+++ b/media/base/video_frame.h
@@ -355,6 +355,15 @@ class MEDIA_EXPORT VideoFrame : public base::RefCountedThreadSafe<VideoFrame> {
   CVPixelBufferRef cv_pixel_buffer() const;
 #endif
 
+  // Sets the mailbox release callback.
+  //
+  // The callback may be run from ANY THREAD, and so it is up to the client to
+  // ensure thread safety.
+  //
+  // WARNING: This method is not thread safe; it should only be called if you
+  // are still the only owner of this VideoFrame.
+  void SetReleaseMailboxCB(const ReleaseMailboxCB& release_mailbox_cb);
+
   // Adds a callback to be run when the VideoFrame is about to be destroyed.
   // The callback may be run from ANY THREAD, and so it is up to the client to
   // ensure thread safety.  Although read-only access to the members of this