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 484acb6486f6591c6b48225440b8d5f8cc15703b..ef266d1faa195eb64ea1c748493ae71fc6917b98 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -26,7 +26,8 @@ SingleThreadProxy::SingleThreadProxy(LayerTreeHost* layer_tree_host)
       layer_tree_host_(layer_tree_host),
       created_offscreen_context_provider_(false),
       next_frame_is_newly_committed_frame_(false),
-      inside_draw_(false) {
+      inside_draw_(false),
+      can_cancel_commit_(false) {
   TRACE_EVENT0("cc", "SingleThreadProxy::SingleThreadProxy");
   DCHECK(Proxy::IsMainThread());
   DCHECK(layer_tree_host);
@@ -171,6 +172,11 @@ 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.
@@ -219,6 +225,7 @@ void SingleThreadProxy::DoCommit(scoped_ptr<ResourceUpdateQueue> queue) {
 
 void SingleThreadProxy::SetNeedsCommit() {
   DCHECK(Proxy::IsMainThread());
+  can_cancel_commit_ = false;
   layer_tree_host_->ScheduleComposite();
 }
 
@@ -262,6 +269,9 @@ void SingleThreadProxy::OnCanDrawStateChanged(bool can_draw) {
 }
 
 void SingleThreadProxy::SetNeedsRedrawOnImplThread() {
+  // Since commit+draw happens in a single pass for this proxy,
+  // this also prevents cancelling a commit.
+  can_cancel_commit_ = false;
   layer_tree_host_->ScheduleComposite();
 }
 
@@ -400,11 +410,21 @@ bool SingleThreadProxy::CommitAndComposite(
         ->UnlinkAndClearEvictedBackings();
   }
 
+  // Reset this flag for the next commit.  It's possible that UpdateLayers
+  // will force another commit, so record this flag.
+  bool can_cancel_this_commit = can_cancel_commit_ && !for_readback;
+  can_cancel_commit_ = true;
+
   scoped_ptr<ResourceUpdateQueue> queue =
       make_scoped_ptr(new ResourceUpdateQueue);
-  layer_tree_host_->UpdateLayers(
+  bool updated = layer_tree_host_->UpdateLayers(
       queue.get(), layer_tree_host_impl_->memory_allocation_limit_bytes());
 
+  if (!updated && can_cancel_this_commit) {
+    TRACE_EVENT0("cc", "EarlyOut_NoUpdates");
+    return false;
+  }
+
   layer_tree_host_->WillCommit();
   DoCommit(queue.Pass());
   bool result = DoComposite(offscreen_context_provider,
@@ -413,6 +433,7 @@ bool SingleThreadProxy::CommitAndComposite(
                             for_readback,
                             frame);
   layer_tree_host_->DidBeginFrame();
+
   return result;
 }