Don't populate the external view embedder in PaintContext.

shell/common/rasterizer.cc

@@ -162,8 +162,19 @@ bool Rasterizer::DrawToSurface(flow::LayerTree& layer_tree) {
 
   auto canvas = frame->SkiaCanvas();
 
+  // External view embedding required that the gpu and platform threads are the
+  // same. The dynamic merging of these threads is WIP so for now we don't
+  // populate the view embedder. Once we can merge the threads, we should
+  // populate the view embedded here with surface_->GetExternalViewEmbedder() if
+  // the scene contains an external view (and we can probably assert that the
+  // gpu and platform threads are the same).
+  //
+  // TODO(amirh): populate the view embedder once we dynamically merge the
+  // threads for embedded platform views.
+  auto external_view_embedder = nullptr;
+
   auto compositor_frame = compositor_context_->AcquireFrame(
-      surface_->GetContext(), canvas, surface_->GetExternalViewEmbedder(),
+      surface_->GetContext(), canvas, external_view_embedder,
       surface_->GetRootTransformation(), true);
 
   if (canvas) {