Make OffscreenCanvas animation in sync with its placeholder canvas's parent frame rate

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
index 68d1351e64a3af40a74ff91a8bf00eac10f2c91e..a37e4106b3aacc12cf278c06b95a5f064a29a94d 100644
--- a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
@@ -38,6 +38,7 @@
 #include "core/dom/ExceptionCode.h"
 #include "core/dom/TaskRunnerHelper.h"
 #include "core/fileapi/File.h"
+#include "core/frame/FrameHost.h"
 #include "core/frame/ImageBitmap.h"
 #include "core/frame/LocalFrame.h"
 #include "core/frame/Settings.h"
@@ -56,6 +57,7 @@
 #include "core/layout/LayoutHTMLCanvas.h"
 #include "core/layout/api/LayoutViewItem.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
+#include "core/page/ChromeClient.h"
 #include "core/paint/PaintLayer.h"
 #include "core/paint/PaintTiming.h"
 #include "platform/Histogram.h"
@@ -1423,7 +1425,14 @@ String HTMLCanvasElement::getIdFromControl(const Element* element) {
 
 void HTMLCanvasElement::createLayer() {
   DCHECK(!m_surfaceLayerBridge);
-  m_surfaceLayerBridge = WTF::wrapUnique(new CanvasSurfaceLayerBridge(this));
+  LocalFrame* frame = document().frame();
+  WebLayerTreeView* layerTreeView = nullptr;
+  if (frame && frame->host()) {

[dcheng, 2017/01/20 08:44:12]

No need to check frame->host() as well: if Document::frame() is not null, that
means the frame is attached (which means FrameHost must not be null)

[xlai (Olivia), 2017/01/20 16:36:21]

Done.

+    layerTreeView = frame->host()->chromeClient().getWebLayerTreeView();
+  }
+
+  m_surfaceLayerBridge =
+      WTF::wrapUnique(new CanvasSurfaceLayerBridge(this, layerTreeView));
   // Creates a placeholder layer first before Surface is created.
   m_surfaceLayerBridge->createSolidColorLayer();
 }