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

content/public/browser/render_view_host.h

Index: content/public/browser/render_view_host.h
diff --git a/content/public/browser/render_view_host.h b/content/public/browser/render_view_host.h
index 6a6b34982da73492878165279bcd4d7ee38bfeec..51b3f31d9455a7764963893df5575ee59a6a554f 100644
--- a/content/public/browser/render_view_host.h
+++ b/content/public/browser/render_view_host.h
@@ -91,6 +91,10 @@ class CONTENT_EXPORT RenderViewHost : virtual public RenderWidgetHost {
   // image at that location).
   virtual void CopyImageAt(int x, int y) = 0;
 
+  // Saves the image at location x, y to the disk (if there indeed is an
+  // image at that location).
+  virtual void SaveImageAt(int x, int y) = 0;

[Avi (use Gerrit), 2014/04/30 14:52:28]

I'm feeling weird that we're introducing a "save image" call that's not used for
the usual "save image" mechanism. Are you going to retrofit our existing "save
image" to it?

+
   // Notifies the listener that a directory enumeration is complete.
   virtual void DirectoryEnumerationFinished(
       int request_id,