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

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

 /*
  * Copyright (C) 2008 Apple 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 84 matching lines @@
       IgnoredReasons* = nullptr) const override;
   bool computeAccessibilityIsIgnored(IgnoredReasons* = nullptr) const override;
 
   // Properties of static elements.
   const AtomicString& accessKey() const override;
   RGBA32 computeBackgroundColor() const final;
   RGBA32 color() const final;
   String fontFamily() const final;
   // Font size is in pixels.
   float fontSize() const final;
+  String imageDataUrl(const IntSize& maxSize) const final;
   String text() const override;
   AccessibilityTextDirection textDirection() const final;
   int textLength() const override;
   TextStyle getTextStyle() const final;
   KURL url() const override;
 
   // Inline text boxes.
   void loadInlineTextBoxes() override;
   AXObject* nextOnLine() const override;
   AXObject* previousOnLine() const override;
@@ skipping 102 matching lines @@
   AXRange textControlSelection() const;
   int indexForVisiblePosition(const VisiblePosition&) const;
   AXLayoutObject* getUnignoredObjectFromNode(Node&) const;
 };
 
 DEFINE_AX_OBJECT_TYPE_CASTS(AXLayoutObject, isAXLayoutObject());
 
 }  // namespace blink
 
 #endif  // AXLayoutObject_h