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

content/browser/renderer_host/render_message_filter.h

Index: content/browser/renderer_host/render_message_filter.h
diff --git a/content/browser/renderer_host/render_message_filter.h b/content/browser/renderer_host/render_message_filter.h
index 3c42f53d453ec12bc173a9695bfddf04878e45bc..390860b3a17538b801f5b175a82b2f9fae7f74d0 100644
--- a/content/browser/renderer_host/render_message_filter.h
+++ b/content/browser/renderer_host/render_message_filter.h
@@ -196,7 +196,8 @@ class RenderMessageFilter : public BrowserMessageFilter {
   void OnDownloadUrl(const IPC::Message& message,
                      const GURL& url,
                      const Referrer& referrer,
-                     const base::string16& suggested_name);
+                     const base::string16& suggested_name,
+                     const bool use_prompt);
   void OnCheckNotificationPermission(const GURL& source_origin,
                                      int* permission_level);