Add BorderBoxOutsets method to LayoutBoxModelObject

R=dgrogan@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2878723003
Cr-Commit-Position: refs/heads/master@{#471163}

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

@@ -286,6 +286,11 @@ class CORE_EXPORT LayoutBoxModelObject : public LayoutObject {
   LayoutUnit BorderWidth() const { return BorderLeft() + BorderRight(); }
   LayoutUnit BorderHeight() const { return BorderTop() + BorderBottom(); }
 
+  virtual LayoutRectOutsets BorderBoxOutsets() const {
+    return LayoutRectOutsets(BorderTop(), BorderRight(), BorderBottom(),
+                             BorderLeft());
+  }
+
   // Insets from the border box to the inside of the border.
   LayoutRectOutsets BorderInsets() const {
     return LayoutRectOutsets(-BorderTop(), -BorderRight(), -BorderBottom(),

third_party/WebKit/Source/core/paint/BoxPainter.cpp

@@ -77,9 +77,7 @@ void BoxPainter::PaintBoxDecorationBackground(const PaintInfo& paint_info,
     // The background painting code assumes that the borders are part of the
     // paintRect so we expand the paintRect by the border size when painting the
     // background into the scrolling contents layer.
-    paint_rect.ExpandEdges(layout_box_.BorderTop(), layout_box_.BorderRight(),
-                           layout_box_.BorderBottom(),
-                           layout_box_.BorderLeft());
+    paint_rect.Expand(layout_box_.BorderBoxOutsets());
   } else {
     paint_rect = layout_box_.BorderBoxRect();
   }