cc: remove virtual from Layer::SetContentsOpaque

Any child classes don't override it.
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2624323002
Cr-Commit-Position: refs/heads/master@{#443094}

cc/layers/layer.h

@@ -160,7 +160,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
     return inputs_.background_filters;
   }
 
-  virtual void SetContentsOpaque(bool opaque);
+  void SetContentsOpaque(bool opaque);
   bool contents_opaque() const { return inputs_.contents_opaque; }
 
   void SetPosition(const gfx::PointF& position);