Fixing flakiness of TextureLayerChangeInvisibleMailboxTest

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 0386145c7c55fea08e4f34362dd5f3536b1b1a02..3e307bb7b1a54e5cfd92b0aa1f9d373ae9269d4d 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -425,7 +425,12 @@ void SingleThreadProxy::DidReceiveCompositorFrameAckOnImplThread() {
                "SingleThreadProxy::DidReceiveCompositorFrameAckOnImplThread");
   if (scheduler_on_impl_thread_)
     scheduler_on_impl_thread_->DidReceiveCompositorFrameAck();
-  layer_tree_host_->DidReceiveCompositorFrameAck();
+  // We do a PostTask here because freeing resources in some cases (such as in
+  // TextureLayer) is PostTasked and we want to make sure ack is received after
+  // resources are returned.
+  task_runner_provider_->MainThreadTaskRunner()->PostTask(
+      FROM_HERE, base::Bind(&SingleThreadProxy::DidReceiveCompositorFrameAck,
+                            weak_factory_.GetWeakPtr()));
 }
 
 void SingleThreadProxy::OnDrawForCompositorFrameSink(
@@ -794,4 +799,9 @@ void SingleThreadProxy::DidFinishImplFrame() {
 #endif
 }
 
+void SingleThreadProxy::DidReceiveCompositorFrameAck() {
+  if (layer_tree_host_)

[danakj, 2017/04/04 19:20:54]

This is unusual in this file, the host is only null after Stop() happens. After
which the proxy is destroyed IIRC. Can you explain why it's here?

[Saman Sami, 2017/04/04 19:32:31]

I'm not familiar with lifetime of layer_tree_host_ compared to the proxy so I
thought this check will be useful. I'll get rid of it.

[danakj, 2017/04/04 19:36:21]

I'm quite against branches just-in-case, and want them understood and explained,
because every branch is complexity when someone else reads or wants to change
the code later. Then they'd wonder when layer_tree_host_ is null, and people
will easily cargo-cult it into other places, and it just kinda spreads. Sounds
good to remove then.

[Saman Sami, 2017/04/04 21:39:14]

Done.

+    layer_tree_host_->DidReceiveCompositorFrameAck();
+}
+
 }  // namespace cc