Fix race conditions in window snapshot code.

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 6714503e5c6976e4861c38dba0aec479b8ca764e..faf360f6fc3435c2430164ac0a9458c5947ae357 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -164,6 +164,8 @@ class CC_EXPORT LayerTreeImpl {
   void set_needs_full_tree_sync(bool needs) { needs_full_tree_sync_ = needs; }
   bool needs_full_tree_sync() const { return needs_full_tree_sync_; }
 
+  void ForceRedrawNextActivation() { next_activation_forces_redraw_ = true; }
+
   void set_ui_resource_request_queue(const UIResourceRequestQueue& queue);
 
   const LayerImplList& RenderSurfaceLayerList() const;
@@ -261,6 +263,8 @@ class CC_EXPORT LayerTreeImpl {
   // structural differences relative to the active tree.
   bool needs_full_tree_sync_;
 
+  bool next_activation_forces_redraw_;
+
   ui::LatencyInfo latency_info_;
 
   UIResourceRequestQueue ui_resource_request_queue_;