Use nine-patch resource for drawing Aura overlay scrollbar thumb.

third_party/WebKit/Source/platform/exported/WebScrollbarThemePainter.cpp

Index: third_party/WebKit/Source/platform/exported/WebScrollbarThemePainter.cpp
diff --git a/third_party/WebKit/Source/platform/exported/WebScrollbarThemePainter.cpp b/third_party/WebKit/Source/platform/exported/WebScrollbarThemePainter.cpp
index 0f9a326966d4e4eed78126f52bcb4923425b847f..7dce90e628cf7f327c2065078ac18fdf7472926d 100644
--- a/third_party/WebKit/Source/platform/exported/WebScrollbarThemePainter.cpp
+++ b/third_party/WebKit/Source/platform/exported/WebScrollbarThemePainter.cpp
@@ -167,4 +167,8 @@ bool WebScrollbarThemePainter::thumbNeedsRepaint() const {
   return m_scrollbar->thumbNeedsRepaint();
 }
 
+bool WebScrollbarThemePainter::usesNinePatchThumbResource() const {
+  return m_theme->usesNinePatchThumbResource();
+}
+
 }  // namespace blink