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

cc/trees/layer_tree_impl.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_LAYER_TREE_IMPL_H_
 #define CC_TREES_LAYER_TREE_IMPL_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 267 matching lines @@
   // Distribute the rool scroll between outer and inner viewport scroll layer.
   // The outer viewport scroll layer scrolls first.
   void DistributeRootScrollOffset();
 
   void ApplyScroll(LayerImpl* layer, ScrollState* scroll_state) {
     layer_tree_host_impl_->ApplyScroll(layer, scroll_state);
   }
 
   // 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

[danakj, 2015/08/27 17:52:54]

Update this comment then about the pending tree please

[Tobias Sargeant, 2015/08/28 10:58:09]

Done.

+  // 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);
   void BreakSwapPromises(SwapPromise::DidNotSwapReason reason);
 
   void DidModifyTilePriorities();
 
   ResourceId ResourceIdForUIResource(UIResourceId uid) const;
   void ProcessUIResourceRequestQueue();
 
@@ skipping 139 matching lines @@
 
   // In impl-side painting mode, this is true when the tree may contain
   // structural differences relative to the active tree.
   bool needs_full_tree_sync_;
 
   bool next_activation_forces_redraw_;
 
   bool has_ever_been_drawn_;
 
   ScopedPtrVector<SwapPromise> swap_promise_list_;
+  ScopedPtrVector<SwapPromise> pinned_swap_promise_list_;
 
   UIResourceRequestQueue ui_resource_request_queue_;
 
   int render_surface_layer_list_id_;
 
   // Whether or not Blink's viewport size was shrunk by the height of the top
   // controls at the time of the last layout.
   bool top_controls_shrink_blink_size_;
 
   float top_controls_height_;
 
   // The amount that the top controls are shown from 0 (hidden) to 1 (fully
   // shown).
   scoped_refptr<SyncedTopControls> top_controls_shown_ratio_;
 
   scoped_ptr<PendingPageScaleAnimation> pending_page_scale_animation_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_IMPL_H_