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

third_party/WebKit/public/platform/WebScrollbarThemePainter.h

Index: third_party/WebKit/public/platform/WebScrollbarThemePainter.h
diff --git a/third_party/WebKit/public/platform/WebScrollbarThemePainter.h b/third_party/WebKit/public/platform/WebScrollbarThemePainter.h
index 665e746c760580fb788d3143ee823b9e8d610c6a..b46df87ae6e077271bd28207536548e7a9c3fcc0 100644
--- a/third_party/WebKit/public/platform/WebScrollbarThemePainter.h
+++ b/third_party/WebKit/public/platform/WebScrollbarThemePainter.h
@@ -28,13 +28,14 @@
 
 #include "public/platform/WebCanvas.h"
 #include "public/platform/WebPrivatePtr.h"
+#include "public/platform/WebRect.h"
+#include "public/platform/WebSize.h"
 
 namespace blink {
 
 class ScrollbarTheme;
 class Scrollbar;
 class WebScrollbar;
-struct WebRect;
 
 class WebScrollbarThemePainter {
  public:
@@ -71,6 +72,8 @@ class WebScrollbarThemePainter {
   BLINK_PLATFORM_EXPORT bool trackNeedsRepaint() const;
   BLINK_PLATFORM_EXPORT bool thumbNeedsRepaint() const;
 
+  BLINK_PLATFORM_EXPORT bool usesNinePatchThumbResource() const;
+
 #if INSIDE_BLINK
   BLINK_PLATFORM_EXPORT WebScrollbarThemePainter(ScrollbarTheme&,
                                                  Scrollbar&,