Handle exception when user call canvas.transferControlToOffscreen() twice

third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.cpp

Index: third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.cpp
diff --git a/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.cpp b/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.cpp
index 4c30861ef7c573338193634985326ede43e9da06..bcd0143e15a4e838b9a3637845c2de256e5e32c1 100644
--- a/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.cpp
+++ b/third_party/WebKit/Source/modules/canvas/HTMLCanvasElementModule.cpp
@@ -37,6 +37,13 @@ void HTMLCanvasElementModule::getContext(
 OffscreenCanvas* HTMLCanvasElementModule::transferControlToOffscreen(
     HTMLCanvasElement& canvas,
     ExceptionState& exceptionState) {
+  if (!!canvas.surfaceLayerBridge()) {

[Justin Novosad, 2016/11/02 20:00:11]

No need for !!

[xlai (Olivia), 2016/11/02 20:07:50]

Done.

+    exceptionState.throwDOMException(
+        InvalidStateError,
+        "Cannot transfer control from a canvas for more than one time.");
+    return nullptr;
+  }
+
   if (!canvas.createSurfaceLayer()) {
     exceptionState.throwDOMException(
         V8Error,