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..628206e6d0a8aa1df4ce28ef3614eace4521f04d 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 +1425,17 @@ 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;
+  // TODO(xlai): Ensure OffscreenCanvas commit() is still functional when a
+  // frame-less HTML canvas's document is reparenting under another frame.
+  // See crbug.com/683172.
+  if (frame) {
+    layerTreeView = frame->host()->chromeClient().getWebLayerTreeView();

[dcheng, 2017/01/23 21:53:32]

Sorry, one more dumb question: why do we call createLayer() still if the
document is detached? I read the MDN example:

https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/transferCo...

which appears to create a detached canvas element. But it's not clear to me what
effect committing in a detached canvas element would have. Is this something
that's supposed to work in the future?

[xlai (Olivia), 2017/01/24 20:54:12]

You are right, it doesn't make sense to have a layer for a frameless canvas.

I have done some changes so that for a frameless canvas, there is no mojo
channels built for this 
edge case. However, since a frameless canvas still needs to be used as
image source, the commit() call will still need to trigger a post of image from
worker thread to main
thread; no browser thread would be involved in this process though.

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