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

ui/native_theme/native_theme_win.cc

Index: ui/native_theme/native_theme_win.cc
diff --git a/ui/native_theme/native_theme_win.cc b/ui/native_theme/native_theme_win.cc
index e0d92f10cee0223caa9fafe7d2edbca9a083158c..8f7502cd5880abda413c4115a5b0464c97a84a98 100644
--- a/ui/native_theme/native_theme_win.cc
+++ b/ui/native_theme/native_theme_win.cc
@@ -660,6 +660,20 @@ SkColor NativeThemeWin::GetSystemColor(ColorId color_id) const {
   return GetAuraColor(color_id, this);
 }
 
+bool NativeThemeWin::SupportsNinePatch(Part part) const {
+  return false;

[sadrul, 2017/03/01 15:38:55]

We are using the same compositor on windows. Why would it not support nine-patch
layer?

[bokan, 2017/03/01 16:22:45]

Only the scrollbar thumb for overlay scrollbars will now be a nine-patch.
That'll only be used on CrOS in stable, but even on windows if enabled via flag,
the overlay scrollbar thumb is painted by native_theme_aura (as is all Web
content). I've added a comment here to make that clearer.

+}
+
+gfx::Size NativeThemeWin::GetNinePatchCanvasSize(Part part) const {
+  NOTREACHED() << "NativeThemeWin doesn't support nine-patch resources.";
+  return gfx::Size();
+}
+
+gfx::Rect NativeThemeWin::GetNinePatchAperture(Part part) const {
+  NOTREACHED() << "NativeThemeWin doesn't support nine-patch resources.";
+  return gfx::Rect();
+}
+
 void NativeThemeWin::PaintIndirect(cc::PaintCanvas* destination_canvas,
                                    Part part,
                                    State state,