Fix race conditions in window snapshot code.

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 8c87b75270aea760c60a1bbd93337417b97366ff..471a1331e6d5236ecd490dfe08792b53479e10f0 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -40,7 +40,8 @@ LayerTreeImpl::LayerTreeImpl(LayerTreeHostImpl* layer_tree_host_impl)
       contents_textures_purged_(false),
       viewport_size_invalid_(false),
       needs_update_draw_properties_(true),
-      needs_full_tree_sync_(true) {
+      needs_full_tree_sync_(true),
+      next_activation_forces_redraw_(false) {
 }
 
 LayerTreeImpl::~LayerTreeImpl() {
@@ -111,6 +112,11 @@ void LayerTreeImpl::PushPropertiesTo(LayerTreeImpl* target_tree) {
   // The request queue should have been processed and does not require a push.
   DCHECK_EQ(ui_resource_request_queue_.size(), 0u);
 
+  if (next_activation_forces_redraw_) {
+    layer_tree_host_impl_->SetFullRootLayerDamage();
+    next_activation_forces_redraw_ = false;
+  }
+
   target_tree->SetLatencyInfo(latency_info_);
   latency_info_.Clear();
   target_tree->SetPageScaleFactorAndLimits(