[cc] Fix CompositorFrameSinkSupport BeginFrameAck interface.

third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp

Index: third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
diff --git a/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp b/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
index a41f0c69a83ff962d9490753f956682b135ca320..de6f29c6c19b0bb5aa73c49c5985e49343f6cac9 100644
--- a/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
+++ b/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
@@ -246,6 +246,8 @@ ScriptPromise OffscreenCanvas::commit(RefPtr<StaticBitmapImage> image,
     m_overdrawFrame = nullptr;
     m_commitPromiseResolver = ScriptPromiseResolver::create(scriptState);
     m_commitPromiseResolver->keepAliveWhilePending();
+    // TODO(eseckler): OffscreenCanvas shouldn't dispatch CompositorFrames
+    // without a prior BeginFrame.
     doCommit(std::move(image), isWebGLSoftwareRendering);
   }
   return m_commitPromiseResolver->promise();
@@ -259,6 +261,8 @@ void OffscreenCanvas::doCommit(RefPtr<StaticBitmapImage> image,
 }
 
 void OffscreenCanvas::beginFrame() {
+  // TODO(eseckler): beginFrame() shouldn't be used as confirmation of
+  // CompositorFrame activation.
   if (m_overdrawFrame) {
     // if we have an overdraw backlog, push the frame from the backlog
     // first and save the promise resolution for later.