Fix UB with LayerTreeImpl::PassSwapPromises usage

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 8e0493a8c1af61f5f87ab840f4ae5be1a468df18..63d11cae272a991925f485a9201f618a5d3f36d7 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -511,7 +511,11 @@ void LayerTreeHost::FinishCommitOnImplThread(LayerTreeHostImpl* host_impl) {
   if (sync_tree->IsActiveTree())
     sync_tree->elastic_overscroll()->PushPendingToActive();
 
-  sync_tree->PassSwapPromises(std::move(swap_promise_list_));

[brianderson, 2016/08/15 17:05:55]

Thanks for catching this!

It would be more straightforward to call swap_promise_list_.clear() after this
line which, looking online, seems to be just as valid.

[svartmetal, 2016/08/15 19:04:33]

Fixed. It's OK to use std::vector::clear since it has no preconditions. On the
other hand, it leaves the capacity() of the vector unchanged (maybe it is better
to use assignment operator instead).

+  {
+    std::vector<std::unique_ptr<SwapPromise>> to_pass;
+    swap_promise_list_.swap(to_pass);
+    sync_tree->PassSwapPromises(std::move(to_pass));
+  }
 
   sync_tree->set_top_controls_shrink_blink_size(
       top_controls_shrink_blink_size_);