--- a/dom/media/VideoFrameContainer.h
+++ b/dom/media/VideoFrameContainer.h
@@ -51,16 +51,25 @@ public:
     SetCurrentFrames(aIntrinsicSize, nsTArray<ImageContainer::NonOwningImage>());
   }
 
   void ClearCurrentFrame();
   // Time in seconds by which the last painted video frame was late by.
   // E.g. if the last painted frame should have been painted at time t,
   // but was actually painted at t+n, this returns n in seconds. Threadsafe.
   double GetFrameDelay();
+
+  // Returns a new frame ID for SetCurrentFrames().  The client must either
+  // call this on only one thread or provide barriers.  Do not use together
+  // with SetCurrentFrame().
+  ImageContainer::FrameID NewFrameID()
+  {
+    return ++mFrameID;
+  }
+
   // Call on main thread
   enum {
     INVALIDATE_DEFAULT,
     INVALIDATE_FORCE
   };
   void Invalidate() { InvalidateWithFlags(INVALIDATE_DEFAULT); }
   B2G_ACL_EXPORT void InvalidateWithFlags(uint32_t aFlags);
   B2G_ACL_EXPORT ImageContainer* GetImageContainer();
@@ -78,18 +87,18 @@ protected:
   // mMutex protects all the fields below.
   Mutex mMutex;
   // The intrinsic size is the ideal size which we should render the
   // ImageContainer's current Image at.
   // This can differ from the Image's actual size when the media resource
   // specifies that the Image should be stretched to have the correct aspect
   // ratio.
   gfxIntSize mIntrinsicSize;
-  // For SetCurrentFrame callers we maintain our own mFrameID which is auto-
-  // incremented at every SetCurrentFrame.
+  // We maintain our own mFrameID which is auto-incremented at every
+  // SetCurrentFrame() or NewFrameID() call.
   ImageContainer::FrameID mFrameID;
   // True when the intrinsic size has been changed by SetCurrentFrame() since
   // the last call to Invalidate().
   // The next call to Invalidate() will recalculate
   // and update the intrinsic size on the element, request a frame reflow and
   // then reset this flag.
   bool mIntrinsicSizeChanged;
   // True when the Image size has changed since the last time Invalidate() was