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 f7f77fad21c03aee84e63185e7b6b41f81886288..de43463de16c2f8099ede83e2a78318f08c3d422 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -1523,7 +1523,11 @@ 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);
   }