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

public/web/WebView.h

Index: public/web/WebView.h
diff --git a/public/web/WebView.h b/public/web/WebView.h
index a5669eda463062f9281eff403cc2b0003c9ea76e..14d492cdee8424ce6c2004f11ad4edc45c18997c 100644
--- a/public/web/WebView.h
+++ b/public/web/WebView.h
@@ -338,6 +338,10 @@ public:
     // WebView (if there is such an image)
     virtual void copyImageAt(const WebPoint&) = 0;
 
+    // Save as the image located at a particular point in the
+    // WebView (if there is such an image)
+    virtual void saveImageAt(const WebPoint&) = 0;
+
     // Notifies the WebView that a drag has terminated.
     virtual void dragSourceEndedAt(
         const WebPoint& clientPoint, const WebPoint& screenPoint,