Reland "Shouldn't access LayerTreeTest::layer_tree_host on impl thread"

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 92b7b68157142de9f282d6af92b9a9f8034a0c7b..0cf6e477b3eb5705065c96d1e35a6fcf291da51d 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -96,7 +96,7 @@ void SingleThreadProxy::SetLayerTreeHostClientReady() {
 }
 
 void SingleThreadProxy::SetVisible(bool visible) {
-  DebugScopedSetImplThread impl(this);
+  DebugScopedSetImplThreadAndMainThreadBlocked impl(this);
   layer_tree_host_impl_->SetVisible(visible);
 }
 
@@ -175,9 +175,7 @@ void SingleThreadProxy::DoCommit(scoped_ptr<ResourceUpdateQueue> queue) {
   DCHECK(Proxy::IsMainThread());
   // Commit immediately.
   {
-    DebugScopedSetMainThreadBlocked mainThreadBlocked(this);
-    DebugScopedSetImplThread impl(this);
-
+    DebugScopedSetImplThreadAndMainThreadBlocked impl_with_main_blocked(this);
     RenderingStatsInstrumentation* stats_instrumentation =
         layer_tree_host_->rendering_stats_instrumentation();
     base::TimeTicks start_time = stats_instrumentation->StartRecording();