Rewrite Layer::SetScrollOffset{FromImplSide} property tree fast-path

cc/layers/layer.h

 // Copyright 2010 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_LAYERS_LAYER_H_
 #define CC_LAYERS_LAYER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 487 matching lines @@
   void RemoveFromClipTree();
 
   // When we detach or attach layer to new LayerTreeHost, all property trees'
   // indices becomes invalid.
   void InvalidatePropertyTreesIndices();
 
   // This is set whenever a property changed on layer that affects whether this
   // layer should own a property tree node or not.
   void SetPropertyTreesNeedRebuild();
 
+  // Fast-path for |SetScrollOffset| and |SetScrollOffsetFromImplSide| to
+  // directly update scroll offset property values in the property tree without
+  // needing a full tree rebuild. Returns true if the property tree nodes were
+  // updated, false otherwise.
+  bool UpdateExistingScrollOffsetPropertyTreeNodes(const gfx::ScrollOffset&);

[wkorman, 2017/05/11 17:42:13]

What about just "UpdateScrollOffset"? The rest seems implicit.

[enne (OOO), 2017/05/11 17:44:28]

I assume this dance here is because you can SetScrollOffset before the first
commit including this layer where property trees are built?

This is just a minor nit, but I'm not sure why the caller needs to care about
the return value.  For simplicity, could this function just be void, and just
DCHECK that it needs a rebuild internally instead of returning false?

[pdr., 2017/05/11 18:03:37]

Good idears. Done.

+
   // Encapsulates all data, callbacks or interfaces received from the embedder.
   // TODO(khushalsagar): This is only valid when PropertyTrees are built
   // internally in cc. Update this for the SPv2 path where blink generates
   // PropertyTrees.
   struct Inputs {
     explicit Inputs(int layer_id);
     ~Inputs();
 
     int layer_id;
 
@@ skipping 111 matching lines @@
   // These all act like draw properties, so don't need push properties.
   gfx::Rect visible_layer_rect_;
   size_t num_unclipped_descendants_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_H_