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..5b69fb7a105392ed06c3981316082a0aba08dafd 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -285,8 +285,26 @@ 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. Pinned swap promises
+  // may only be queued on the active tree.
+  //
+  // An active tree pinned swap promise will see only DidSwap() or
+  // DidNotSwap(SWAP_FAILS). No DidActivate() will be seen because
+  // that has already happened prior to queueing of the swap promise.
+  //
+  // Pinned active tree swap promises will not be broken prematurely
+  // on the active tree if a new tree is activated.
+  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 +464,7 @@ 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_;