Re-land: cc: Implement shared worker contexts.

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 48604f7e9760172cf3d48f6f48ed22696201e318..c9f76f769ab8acac7a709ea155c8882344969ea9 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -388,7 +388,9 @@ void LayerTreeHost::SetOutputSurface(scoped_ptr<OutputSurface> surface) {
   DCHECK(output_surface_lost_);
   DCHECK(surface);
 
-  proxy_->SetOutputSurface(surface.Pass());
+  DCHECK(!new_output_surface_);
+  new_output_surface_ = surface.Pass();
+  proxy_->SetOutputSurface(new_output_surface_.get());
 }
 
 void LayerTreeHost::RequestNewOutputSurface() {
@@ -396,12 +398,16 @@ void LayerTreeHost::RequestNewOutputSurface() {
 }
 
 void LayerTreeHost::DidInitializeOutputSurface() {
+  DCHECK(new_output_surface_);
   output_surface_lost_ = false;
   client_->DidInitializeOutputSurface();
+  current_output_surface_ = new_output_surface_.Pass();
 }
 
 void LayerTreeHost::DidFailToInitializeOutputSurface() {
   DCHECK(output_surface_lost_);
+  DCHECK(new_output_surface_);
+  new_output_surface_ = nullptr;
   client_->DidFailToInitializeOutputSurface();
 }