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

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index f1d8c87efc15d6463ca8520a8aca8d9d2313427e..dbd3a620a1ff51554ff1b901af79b9480006275c 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -1537,7 +1537,11 @@ void RenderFrameImpl::loadURLExternally(
   if (policy == blink::WebNavigationPolicyDownload) {
     render_view_->Send(new ViewHostMsg_DownloadUrl(render_view_->GetRoutingID(),
                                                    request.url(), referrer,
-                                                   suggested_name));
+                                                   suggested_name, false));
+  } else if (policy == blink::WebNavigationPolicyDownloadTo) {
+    render_view_->Send(new ViewHostMsg_DownloadUrl(render_view_->GetRoutingID(),
+                                                   request.url(), referrer,
+                                                   suggested_name, true));
   } else {
     OpenURL(frame, request.url(), referrer, policy);
   }