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 2572cd6df23fdc743abdefbf46b397f044c33c3b..860c2159d098777b50de852de21932558b7d8e9f 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 +398,20 @@ void LayerTreeHost::RequestNewOutputSurface() {
 }
 
 void LayerTreeHost::DidInitializeOutputSurface() {
+  DCHECK(new_output_surface_);
   output_surface_lost_ = false;
+  current_output_surface_ = new_output_surface_.Pass();
   client_->DidInitializeOutputSurface();
 }
 
 void LayerTreeHost::DidFailToInitializeOutputSurface() {
   DCHECK(output_surface_lost_);
+  DCHECK(new_output_surface_);
+  // Note: It is safe to drop all output surface references here as
+  // LayerTreeHostImpl will not keep a pointer to either the old or
+  // new output surface after failing to initialize the new one.
+  current_output_surface_ = nullptr;
+  new_output_surface_ = nullptr;
   client_->DidFailToInitializeOutputSurface();
 }