Make OffscreenCanvas an EventTarget

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 94cbf94f568d6c2c2578b12199da58119e0b4067..d5517df272903e2ee0f9d9d89a47f66c86187ec7 100644
--- a/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
+++ b/third_party/WebKit/Source/core/offscreencanvas/OffscreenCanvas.cpp
@@ -237,8 +237,13 @@ ScriptPromise OffscreenCanvas::convertToBlob(ScriptState* scriptState,
   return resolver->promise();
 }
 
+ExecutionContext* OffscreenCanvas::getExecutionContext() const {
+  return m_context->getExecutionContext();
+}
+
 DEFINE_TRACE(OffscreenCanvas) {
   visitor->trace(m_context);
+  EventTarget::trace(visitor);
 }
 
 }  // namespace blink