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

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 581271731c5ad90510b582bc335461e999073947..a29bf7eba7e4a6b7d7e5ab6313fc3d55e5f1d237 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -505,6 +505,12 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
   // 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