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 6b78b257ab7a4d1bfced155e8272429e20a2a72b..5045ec1076e566676c5e55700ffbcb05e52c8d42 100644
--- a/cc/test/layer_tree_test.cc
+++ b/cc/test/layer_tree_test.cc
@@ -413,6 +413,13 @@ 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);
 
@@ -557,6 +564,13 @@ 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;