Implement "Save image as" for canvas (blink side).

public/web/WebContextMenuData.h

 /*
  * Copyright (C) 2009, 2012 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 37 matching lines @@
 struct WebContextMenuData {
     enum MediaType {
         // No special node is in context.
         MediaTypeNone,
         // An image node is selected.
         MediaTypeImage,
         // A video node is selected.
         MediaTypeVideo,
         // An audio node is selected.
         MediaTypeAudio,
+        // A canvas node is selected.
+        MediaTypeCanvas,
         // A file node is selected.
         MediaTypeFile,
         // A plugin node is selected.
         MediaTypePlugin,
         MediaTypeLast = MediaTypePlugin
     };
     // The type of media the context menu is being invoked on.
     MediaType mediaType;
 
     // The x and y position of the mouse pointer (relative to the webview).
@@ skipping 106 matching lines @@
         , isEditable(false)
         , writingDirectionDefault(CheckableMenuItemDisabled)
         , writingDirectionLeftToRight(CheckableMenuItemEnabled)
         , writingDirectionRightToLeft(CheckableMenuItemEnabled)
         , editFlags(0) { }
 };
 
 } // namespace blink
 
 #endif