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

third_party/WebKit/public/platform/WebScrollbarThemeGeometry.h

Index: third_party/WebKit/public/platform/WebScrollbarThemeGeometry.h
diff --git a/third_party/WebKit/public/platform/WebScrollbarThemeGeometry.h b/third_party/WebKit/public/platform/WebScrollbarThemeGeometry.h
index a2c89794a39a6cc88129b59c1bb4fb16a5b72be8..c6291071f83c19e843d0ebef80abab956e252e6a 100644
--- a/third_party/WebKit/public/platform/WebScrollbarThemeGeometry.h
+++ b/third_party/WebKit/public/platform/WebScrollbarThemeGeometry.h
@@ -27,6 +27,7 @@
 #define WebScrollbarThemeGeometry_h
 
 #include "WebRect.h"
+#include "WebSize.h"
 
 namespace blink {
 
@@ -44,6 +45,8 @@ class BLINK_PLATFORM_EXPORT WebScrollbarThemeGeometry {
   virtual WebRect backButtonEndRect(WebScrollbar*) = 0;
   virtual WebRect forwardButtonStartRect(WebScrollbar*) = 0;
   virtual WebRect forwardButtonEndRect(WebScrollbar*) = 0;
+  virtual WebSize ninePatchThumbCanvasSize(WebScrollbar*) = 0;
+  virtual WebRect ninePatchThumbAperture(WebScrollbar*) = 0;
 };
 
 }  // namespace blink