Queue latency info swap promises in a separate already-active queue.

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 47b6351ca6f8d7bdc1b503cc2ec77a5e63d4afc6..176624316a04901be8d374a17d54b890f2eb3463 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -285,8 +285,30 @@ class CC_EXPORT LayerTreeImpl {
 
   // Call this function when you expect there to be a swap buffer.
   // See swap_promise.h for how to use SwapPromise.
+  //
+  // A swap promise queued by QueueSwapPromise travels with the layer
+  // information currently associated with the tree. For example, when
+  // a pending tree is activated, the swap promise is passed to the
+  // active tree along with the layer information. Similarly, when a
+  // new activation overwrites layer information on the active tree,
+  // queued swap promises are broken.
   void QueueSwapPromise(scoped_ptr<SwapPromise> swap_promise);
 
+  // Queue a swap promise, pinned to this tree. A pinned swap promise
+  // will not be passed from the pending tree to the active tree upon
+  // activation, and will not be broken prematurely on the active tree
+  // if a new tree is activated.
+  //
+  // Thus, a pinned swap promise on a pending tree will only see
+  // DidActivate() or DidNotSwap(ACTIVATION_FAILS). It is not passed
+  // to the active tree, so it will not swap.
+  //
+  // A pinned swap promise on the active tree will either see only
+  // DidSwap() or DidNotSwap(SWAP_FAILS). No DidActivate() will be
+  // seen because that has already happened prior to queueing of the
+  // swap promise.
+  void QueuePinnedSwapPromise(scoped_ptr<SwapPromise> swap_promise);
+
   // Take the |new_swap_promise| and append it to |swap_promise_list_|.
   void PassSwapPromises(ScopedPtrVector<SwapPromise>* new_swap_promise);
   void FinishSwapPromises(CompositorFrameMetadata* metadata);
@@ -446,6 +468,7 @@ class CC_EXPORT LayerTreeImpl {
   bool has_ever_been_drawn_;
 
   ScopedPtrVector<SwapPromise> swap_promise_list_;
+  ScopedPtrVector<SwapPromise> pinned_swap_promise_list_;
 
   UIResourceRequestQueue ui_resource_request_queue_;