Add support for retrieving image thumbnails as part of the accessibility tree.

third_party/WebKit/Source/modules/accessibility/AXObject.h

 /*
  * Copyright (C) 2008, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nuanti Ltd.
  *
  * 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 747 matching lines @@
   // Used by objects of role ColorWellRole.
   virtual RGBA32 colorValue() const { return Color::transparent; }
   virtual bool canvasHasFallbackContent() const { return false; }
   virtual String fontFamily() const { return nullAtom; }
   // Font size is in pixels.
   virtual float fontSize() const { return 0.0f; }
   // Value should be 1-based. 0 means not supported.
   virtual int headingLevel() const { return 0; }
   // Value should be 1-based. 0 means not supported.
   virtual unsigned hierarchicalLevel() const { return 0; }
+  // Return the content of an image or canvas as an image data url in
+  // PNG format. If |maxSize| is not empty and if the image is larger than
+  // those dimensions, the image will be resized proportionally first to fit.
+  virtual String imageDataUrl(const IntSize& maxSize) const { return nullAtom; }
   virtual AccessibilityOrientation orientation() const;
   virtual String text() const { return String(); }
   virtual AccessibilityTextDirection textDirection() const {
     return AccessibilityTextDirectionLTR;
   }
   virtual int textLength() const { return 0; }
   virtual TextStyle getTextStyle() const { return TextStyleNone; }
   virtual KURL url() const { return KURL(); }
 
   // Load inline text boxes for just this node, even if
@@ skipping 286 matching lines @@
   static unsigned s_numberOfLiveAXObjects;
 };
 
 #define DEFINE_AX_OBJECT_TYPE_CASTS(thisType, predicate)           \
   DEFINE_TYPE_CASTS(thisType, AXObject, object, object->predicate, \
                     object.predicate)
 
 }  // namespace blink
 
 #endif  // AXObject_h