De-virtualize LayoutBoxModelObject::PaddingOutsets()

Not overriden in any derived classes.

Change-Id: Ic31257df96803ea205834253385d760102a62f86
Reviewed-on: https://chromium-review.googlesource.com/808725Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521719}

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h

@@ -274,7 +274,7 @@ class CORE_EXPORT LayoutBoxModelObject : public LayoutObject {
                              BorderLeft());
   }
 
-  virtual LayoutRectOutsets PaddingOutsets() const {
+  LayoutRectOutsets PaddingOutsets() const {
     return LayoutRectOutsets(PaddingTop(), PaddingRight(), PaddingBottom(),
                              PaddingLeft());
   }