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

third_party/WebKit/public/platform/WebThemeEngine.h

Index: third_party/WebKit/public/platform/WebThemeEngine.h
diff --git a/third_party/WebKit/public/platform/WebThemeEngine.h b/third_party/WebKit/public/platform/WebThemeEngine.h
index 2387f462111bc2bf0ed1918521502caf8db4fd4b..51d6d1a5286f32ab169f5561e38231988e9fcf5a 100644
--- a/third_party/WebKit/public/platform/WebThemeEngine.h
+++ b/third_party/WebKit/public/platform/WebThemeEngine.h
@@ -33,13 +33,12 @@
 
 #include "WebCanvas.h"
 #include "WebColor.h"
+#include "WebRect.h"
 #include "WebScrollbarOverlayColorTheme.h"
 #include "WebSize.h"
 
 namespace blink {
 
-struct WebRect;
-
 class WebThemeEngine {
  public:
   // The current state of the associated Part.
@@ -159,6 +158,10 @@ class WebThemeEngine {
   // the track while the height will be the minimum height.
   virtual WebSize getSize(Part) { return WebSize(); }
 
+  virtual bool supportsNinePatch(Part) const { return false; }
+  virtual WebSize ninePatchCanvasSize(Part) const { return WebSize(); }
+  virtual WebRect ninePatchAperture(Part) const { return WebRect(); }
+
   struct ScrollbarStyle {
     int thumbThickness;
     int scrollbarMargin;