[cc] Fix CompositorFrameSinkSupport BeginFrameAck interface.

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/offscreencanvas/OffscreenCanvas.h"
 
 #include "core/dom/ExceptionCode.h"
 #include "core/fileapi/Blob.h"
 #include "core/frame/ImageBitmap.h"
 #include "core/html/ImageData.h"
@@ skipping 228 matching lines @@
   getOrCreateFrameDispatcher()->setNeedsBeginFrame(true);
   if (m_commitPromiseResolver) {
     if (image) {
       m_overdrawFrame = std::move(image);
       m_overdrawFrameIsWebGLSoftwareRendering = isWebGLSoftwareRendering;
     }
   } else {
     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();
 }
 
 void OffscreenCanvas::doCommit(RefPtr<StaticBitmapImage> image,
                                bool isWebGLSoftwareRendering) {
   double commitStartTime = WTF::monotonicallyIncreasingTime();
   getOrCreateFrameDispatcher()->dispatchFrame(std::move(image), commitStartTime,
                                               isWebGLSoftwareRendering);
 }
 
 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.
     doCommit(std::move(m_overdrawFrame),
              m_overdrawFrameIsWebGLSoftwareRendering);
   } else if (m_commitPromiseResolver) {
     m_commitPromiseResolver->resolve();
     m_commitPromiseResolver.clear();
     // We need to tell parent frame to stop sending signals on begin frame to
     // avoid overhead once we resolve the promise.
@@ skipping 55 matching lines @@
 }
 
 DEFINE_TRACE(OffscreenCanvas) {
   visitor->trace(m_context);
   visitor->trace(m_executionContext);
   visitor->trace(m_commitPromiseResolver);
   EventTargetWithInlineData::trace(visitor);
 }
 
 }  // namespace blink