cc: Allow the main thread to cancel commits

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 426b44f49eebbf18d9a5294a784d4bdbffd7f5e1..2c4d3a89a4c0c7ae7f8c42169ca3242287385cdc 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -171,6 +171,11 @@ void SingleThreadProxy::SetNeedsAnimate() {
   NOTREACHED();
 }
 
+void SingleThreadProxy::SetNeedsUpdateLayers() {
+  DCHECK(Proxy::IsMainThread());
+  layer_tree_host_->ScheduleComposite();
+}
+
 void SingleThreadProxy::DoCommit(scoped_ptr<ResourceUpdateQueue> queue) {
   DCHECK(Proxy::IsMainThread());
   // Commit immediately.
@@ -203,11 +208,12 @@ void SingleThreadProxy::DoCommit(scoped_ptr<ResourceUpdateQueue> queue) {
     layer_tree_host_impl_->CommitComplete();
 
 #ifndef NDEBUG
-    // In the single-threaded case, the scroll deltas should never be
+    // In the single-threaded case, the scale and scroll deltas should never be
     // touched on the impl layer tree.
     scoped_ptr<ScrollAndScaleSet> scroll_info =
         layer_tree_host_impl_->ProcessScrollDeltas();
     DCHECK(!scroll_info->scrolls.size());
+    DCHECK_EQ(1.f, scroll_info->page_scale_delta);
 #endif
 
     base::TimeDelta duration = stats_instrumentation->EndRecording(start_time);