Make stream captures work on canvases that are not in the DOM.

third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef OffscreenCanvasRenderingContext2D_h
 #define OffscreenCanvasRenderingContext2D_h
 
 #include "core/html/canvas/CanvasContextCreationAttributes.h"
 #include "core/html/canvas/CanvasRenderingContext.h"
 #include "core/html/canvas/CanvasRenderingContextFactory.h"
@@ skipping 54 matching lines @@
   bool hasImageBuffer() const final;
   ImageBuffer* imageBuffer() const final;
 
   bool parseColorOrCurrentColor(Color&, const String& colorString) const final;
 
   PaintCanvas* drawingCanvas() const final;
   PaintCanvas* existingDrawingCanvas() const final;
   void disableDeferral(DisableDeferralReason) final;
 
   AffineTransform baseTransform() const final;
-  void didDraw(const SkIRect& dirtyRect) final;
+  void didDraw(const SkIRect& dirtyRect) final;  // overrides
+                                                 // BaseRenderingContext2D and
+                                                 // CanvasRenderingContext
 
   bool stateHasFilter() final;
   sk_sp<SkImageFilter> stateGetFilter() final;
   void snapshotStateForFilter() final {}
 
   void validateStateStack() const final;
 
   bool hasAlpha() const final { return creationAttributes().alpha(); }
   bool isContextLost() const override;
 
@@ skipping 19 matching lines @@
 
 DEFINE_TYPE_CASTS(OffscreenCanvasRenderingContext2D,
                   CanvasRenderingContext,
                   context,
                   context->is2d() && context->offscreenCanvas(),
                   context.is2d() && context.offscreenCanvas());
 
 }  // namespace blink
 
 #endif  // OffscreenCanvasRenderingContext2D_h