Make OffscreenCanvas an EventTarget

third_party/WebKit/Source/core/html/canvas/CanvasRenderingContext.h

Index: third_party/WebKit/Source/core/html/canvas/CanvasRenderingContext.h
diff --git a/third_party/WebKit/Source/core/html/canvas/CanvasRenderingContext.h b/third_party/WebKit/Source/core/html/canvas/CanvasRenderingContext.h
index 8467f9611e13ff66da5ab948abe4da332ec069b7..2f50bb2c444d0bd6e0d07962c5e51fd725f803b4 100644
--- a/third_party/WebKit/Source/core/html/canvas/CanvasRenderingContext.h
+++ b/third_party/WebKit/Source/core/html/canvas/CanvasRenderingContext.h
@@ -176,6 +176,12 @@ class CORE_EXPORT CanvasRenderingContext
     return m_creationAttributes;
   }
 
+  // This method should be called on OffscreenCanvas's context only.
+  virtual ExecutionContext* getExecutionContext() {
+    NOTREACHED();
+    return nullptr;
+  }
+
  protected:
   CanvasRenderingContext(HTMLCanvasElement*,
                          OffscreenCanvas*,