cc: Make async readback path use async glRreadPixels.

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 3b051f7d537121c97966616a654e507994355aa8..7665a38ab56ce3c39bdb62ba6a855290d8bbf099 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -546,12 +546,21 @@ void ThreadProxy::Stop() {
   DCHECK(IsMainThread());
   DCHECK(started_);
 
-  // Synchronously deletes the impl.
+  // Synchronously finishes pending GL operations and deletes the impl.
   {
     DebugScopedSetMainThreadBlocked main_thread_blocked(this);
 
     CompletionEvent completion;
     Proxy::ImplThread()->PostTask(
+        base::Bind(&ThreadProxy::FinishGLOnImplThread,
+                   impl_thread_weak_ptr_,
+                   &completion));
+    completion.Wait();
+  } {
+    DebugScopedSetMainThreadBlocked main_thread_blocked(this);
+
+    CompletionEvent completion;
+    Proxy::ImplThread()->PostTask(
         base::Bind(&ThreadProxy::LayerTreeHostClosedOnImplThread,
                    impl_thread_weak_ptr_,
                    &completion));
@@ -1153,6 +1162,14 @@ void ThreadProxy::InitializeRendererOnImplThread(
   completion->Signal();
 }
 
+void ThreadProxy::FinishGLOnImplThread(CompletionEvent* completion) {
+  TRACE_EVENT0("cc", "ThreadProxy::FinishGLOnImplThread");
+  DCHECK(IsImplThread());
+  if (layer_tree_host_impl_->resource_provider())
+    layer_tree_host_impl_->resource_provider()->Finish();
+  completion->Signal();
+}
+
 void ThreadProxy::LayerTreeHostClosedOnImplThread(CompletionEvent* completion) {
   TRACE_EVENT0("cc", "ThreadProxy::LayerTreeHostClosedOnImplThread");
   DCHECK(IsImplThread());