cc: Commit directly to active tree when using single thread.

cc/trees/thread_proxy.cc

Index: cc/trees/thread_proxy.cc
diff --git a/cc/trees/thread_proxy.cc b/cc/trees/thread_proxy.cc
index 329b3e824d781a74034ce8f2e1208837084f54e5..ab3bb2f4a279d93861720094aff3d9746b609a2d 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -153,6 +153,12 @@ bool ThreadProxy::IsStarted() const {
   return main().started;
 }
 
+bool ThreadProxy::CommitToActiveTree() const {
+  // With ThreadProxy we use a pending tree and activate it once it's ready to
+  // draw.
+  return false;
+}
+
 void ThreadProxy::SetLayerTreeHostClientReady() {
   TRACE_EVENT0("cc", "ThreadProxy::SetLayerTreeHostClientReady");
   Proxy::ImplThreadTaskRunner()->PostTask(