Make WebWidget::compositeAndReadbackAsync() return void.

public/web/WebWidget.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 107 matching lines @@
     // Layout before calling this method. It is okay to call paint
     // multiple times once layout has been called, assuming no other
     // changes are made to the WebWidget (e.g., once events are
     // processed, it should be assumed that another call to layout is
     // warranted before painting again).
     virtual void paint(WebCanvas*, const WebRect& viewPort, PaintOptions = ReadbackFromCompositorIfAvailable) { }
 
     virtual void paintCompositedDeprecated(WebCanvas*, const WebRect&) { }
 
     // The caller is responsible for keeping the WebCompositeAndReadbackAsyncCallback
-    // object alive until it is called.
-    virtual bool compositeAndReadbackAsync(WebCompositeAndReadbackAsyncCallback*) { return false; }
+    // object alive until it is called. This should only be called when
+    // isAcceleratedCompositingActive() is true.
+    virtual void compositeAndReadbackAsync(WebCompositeAndReadbackAsyncCallback*) { }
 
     // Returns true if we've started tracking repaint rectangles.
     virtual bool isTrackingRepaints() const { return false; }
 
     // Indicates that the compositing surface associated with this WebWidget is
     // ready to use.
     virtual void setCompositorSurfaceReady() { }
 
     // Called to inform the WebWidget of a change in theme.
     // Implementors that cache rendered copies of widgets need to re-render
@@ skipping 121 matching lines @@
     // content.
     virtual WebColor backgroundColor() const { return 0xFFFFFFFF; /* SK_ColorWHITE */ }
 
 protected:
     ~WebWidget() { }
 };
 
 } // namespace blink
 
 #endif