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

third_party/WebKit/Source/core/html/HTMLCanvasElement.h

 /*
  * Copyright (C) 2004, 2006, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 152 matching lines @@
   void discardImageBuffer();
 
   bool shouldBeDirectComposited() const;
 
   void prepareSurfaceForPaintingIfNeeded() const;
 
   const AtomicString imageSourceURL() const override;
 
   InsertionNotificationRequest insertedInto(ContainerNode*) override;
 
+  bool isDirty() { return !m_dirtyRect.isEmpty(); }
+
+  void doDeferredPaintInvalidation();
+
+  void finalizeFrame();
+
   // ContextLifecycleObserver and PageVisibilityObserver implementation
   void contextDestroyed(ExecutionContext*) override;
 
   // PageVisibilityObserver implementation
   void pageVisibilityChanged() override;
 
   // CanvasImageSource implementation
   PassRefPtr<Image> getSourceImageForCanvas(SourceImageStatus*,
                                             AccelerationHint,
                                             SnapshotReason,
                                             const FloatSize&) const override;
   bool wouldTaintOrigin(SecurityOrigin*) const override;
   FloatSize elementSize(const FloatSize&) const override;
   bool isCanvasElement() const override { return true; }
   bool isOpaque() const override;
   bool isAccelerated() const override;
   int sourceWidth() override { return m_size.width(); }
   int sourceHeight() override { return m_size.height(); }
 
   // CanvasSurfaceLayerBridgeObserver implementation
   void OnWebLayerReplaced() override;
 
   // ImageBufferClient implementation
   void notifySurfaceInvalid() override;
-  bool isDirty() override { return !m_dirtyRect.isEmpty(); }
   void didDisableAcceleration() override;
-  void didFinalizeFrame() override;
   void restoreCanvasMatrixClipStack(PaintCanvas*) const override;
 
-  void doDeferredPaintInvalidation();
-
   // ImageBitmapSource implementation
   IntSize bitmapSourceSize() const override;
   ScriptPromise createImageBitmap(ScriptState*,
                                   EventTarget&,
                                   Optional<IntRect> cropRect,
                                   const ImageBitmapOptions&,
                                   ExceptionState&) override;
 
   DECLARE_VIRTUAL_TRACE();
 
@@ skipping 98 matching lines @@
   // Used for OffscreenCanvas that controls this HTML canvas element
   std::unique_ptr<CanvasSurfaceLayerBridge> m_surfaceLayerBridge;
 
   int m_numFramesSinceLastRenderingModeSwitch;
   bool m_pendingRenderingModeSwitch;
 };
 
 }  // namespace blink
 
 #endif  // HTMLCanvasElement_h