[RLS] Actually make SA::UsesCompositedScrolling non-virtual.

This was an oversight in a previous CL.

Bug: 818709
Change-Id: I2697d136be7af56e00f342f77451b765a37c8eaa
Reviewed-on: https://chromium-review.googlesource.com/966796Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543795}

third_party/WebKit/Source/platform/scroll/ScrollableArea.h

@@ -277,9 +277,7 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin {
   virtual void RegisterForAnimation() {}
   virtual void DeregisterForAnimation() {}
 
-  virtual bool UsesCompositedScrolling() const {
-    return uses_composited_scrolling_;
-  }
+  bool UsesCompositedScrolling() const { return uses_composited_scrolling_; }
   void SetUsesCompositedScrolling(bool uses_composited_scrolling) {
     uses_composited_scrolling_ = uses_composited_scrolling;
   }