[Blink] Clean-up CompositingInputsUpdater

This CL does some light clean-up to CompositingInputsUpdater.
It is for aesthetic improvement only and should not change behavior.

1. Move AncestorDependentCompositingInputs computation to its own helper
   function.
2. Remove CompositingInputsUpdater::AncestorInfo::has_ancestor_with_clip_path
   which is unused.
3. Add shorthand variable layout_object = layer->GetLayoutObject().

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I745a392b80adedef278fa5ad45436b2c652bf37d
Reviewed-on: https://chromium-review.googlesource.com/950008
Commit-Queue: Tien-Ren Chen <trchen@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541248}

third_party/WebKit/Source/core/paint/compositing/CompositingInputsUpdater.h

@@ -49,10 +49,11 @@ class CompositingInputsUpdater {
     // containing block chain.
     PaintLayer* last_scrolling_ancestor;
     bool has_ancestor_with_clip_related_property;
-    bool has_ancestor_with_clip_path;
   };
 
   void UpdateRecursive(PaintLayer*, UpdateType, AncestorInfo);
+  void UpdateAncestorDependentCompositingInputs(PaintLayer*,
+                                                const AncestorInfo&);
 
   LayoutGeometryMap geometry_map_;
   PaintLayer* root_layer_;