Support single-threaded impl-side painting

cc/trees/single_thread_proxy.cc

Index: cc/trees/single_thread_proxy.cc
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index 1e422cacb7f0e147d251af4429a5c93fa79fb446..720a19938175f96244ed935bb1e730c2fb623ffd 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -465,8 +465,6 @@ void SingleThreadProxy::CompositeImmediately(base::TimeTicks frame_begin_time) {
   TRACE_EVENT0("cc", "SingleThreadProxy::CompositeImmediately");
   DCHECK(Proxy::IsMainThread());
   DCHECK(!layer_tree_host_->output_surface_lost());
-  DCHECK(!layer_tree_host_impl_->settings().impl_side_painting)
-      << "Impl-side painting and synchronous compositing are not supported.";
 
   {
     BeginFrameArgs begin_frame_args(
@@ -482,6 +480,14 @@ void SingleThreadProxy::CompositeImmediately(base::TimeTicks frame_begin_time) {
 
   {
     DebugScopedSetImplThread impl(const_cast<SingleThreadProxy*>(this));
+    if (layer_tree_host_impl_->settings().impl_side_painting) {
+      DCHECK(layer_tree_host_impl_->settings().use_zero_copy);
+      layer_tree_host_impl_->ActivateSyncTree();
+      layer_tree_host_impl_->active_tree()->UpdateDrawProperties();
+      layer_tree_host_impl_->ManageTiles();
+      layer_tree_host_impl_->tile_manager()->WaitForTasksToFinishRunning();
+    }
+
     LayerTreeHostImpl::FrameData frame;
     DoComposite(frame_begin_time, &frame);