Fix race conditions in window snapshot code.

cc/test/layer_tree_test.h

Index: cc/test/layer_tree_test.h
diff --git a/cc/test/layer_tree_test.h b/cc/test/layer_tree_test.h
index c9e14d68b650ed7be13c4d1c7ee6fa6e98781cb2..d7607fb9bcee12f4e6ac2067a470dcd70c4f9ad2 100644
--- a/cc/test/layer_tree_test.h
+++ b/cc/test/layer_tree_test.h
@@ -117,6 +117,7 @@ class LayerTreeTest : public testing::Test, public TestHooks {
   void PostSetNeedsRedrawToMainThread();
   void PostSetNeedsRedrawRectToMainThread(gfx::Rect damage_rect);
   void PostSetVisibleToMainThread(bool visible);
+  void PostSetNextCommitForcesRedrawToMainThread();
 
   void DoBeginTest();
   void Timeout();
@@ -138,6 +139,7 @@ class LayerTreeTest : public testing::Test, public TestHooks {
   void DispatchSetNeedsRedraw();
   void DispatchSetNeedsRedrawRect(gfx::Rect damage_rect);
   void DispatchSetVisible(bool visible);
+  void DispatchSetNextCommitForcesRedraw();
   void DispatchComposite();
   void DispatchDidAddAnimation();