Fix race conditions in window snapshot code.

cc/test/layer_tree_test.cc

Index: cc/test/layer_tree_test.cc
diff --git a/cc/test/layer_tree_test.cc b/cc/test/layer_tree_test.cc
index 87fcb546b3a9883c9106b6849f5255ab7b72919d..6a7cf50c8f384c7852111a8a13d4458b8e6eacd9 100644
--- a/cc/test/layer_tree_test.cc
+++ b/cc/test/layer_tree_test.cc
@@ -414,6 +414,13 @@ void LayerTreeTest::PostSetVisibleToMainThread(bool visible) {
                  visible));
 }
 
+void LayerTreeTest::PostSetNextCommitForcesRedrawToMainThread() {
+  proxy()->MainThreadTaskRunner()->PostTask(
+      FROM_HERE,
+      base::Bind(&LayerTreeTest::DispatchSetNextCommitForcesRedraw,
+                 main_thread_weak_ptr_));
+}
+
 void LayerTreeTest::DoBeginTest() {
   client_ = LayerTreeHostClientForTesting::Create(this);
 
@@ -558,6 +565,13 @@ void LayerTreeTest::DispatchSetVisible(bool visible) {
     ScheduleComposite();
 }
 
+void LayerTreeTest::DispatchSetNextCommitForcesRedraw() {
+  DCHECK(!proxy() || proxy()->IsMainThread());
+
+  if (layer_tree_host_)
+    layer_tree_host_->SetNextCommitForcesRedraw();
+}
+
 void LayerTreeTest::DispatchComposite() {
   scheduled_ = false;