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

ui/native_theme/native_theme_aura.cc

Index: ui/native_theme/native_theme_aura.cc
diff --git a/ui/native_theme/native_theme_aura.cc b/ui/native_theme/native_theme_aura.cc
index 0c3255cb18b6749ccadab206376634289dce1b76..4b12db1a68a7e0dffcd63b14661465a5b5f3944b 100644
--- a/ui/native_theme/native_theme_aura.cc
+++ b/ui/native_theme/native_theme_aura.cc
@@ -317,4 +317,27 @@ gfx::Size NativeThemeAura::GetPartSize(Part part,
   return NativeThemeBase::GetPartSize(part, state, extra);
 }
 
+bool NativeThemeAura::SupportsNinePatch(Part part) const {
+  if (!IsOverlayScrollbarEnabled())
+    return false;
+
+  return part == kScrollbarHorizontalThumb || part == kScrollbarVerticalThumb;
+}
+
+gfx::Size NativeThemeAura::GetNinePatchCanvasSize(Part part) const {
+  DCHECK(SupportsNinePatch(part));
+
+  // 2 pixel border with 1 pixel center patch.

[Evan Stade, 2017/03/01 16:45:49]

2 pixels or 2 dip? Why 2 pixels if OverlayScrollbarStrokeWidth is 1?

[bokan, 2017/03/01 17:02:31]

It's to prevent color interpolation between the patches. The scrollbar stroke is
the outer pixel, the inner pixel should match the color of the inner patch so
that we don't get a smooth transition from the stroke color to the fill color.
Added a clarifying comment.

+  return gfx::Size(5, 5);

[Evan Stade, 2017/03/01 16:45:49]

can you pull out these values into named constants?

[bokan, 2017/03/01 17:02:31]

Done.

+}
+
+gfx::Rect NativeThemeAura::GetNinePatchAperture(Part part) const {
+  DCHECK(SupportsNinePatch(part));
+
+  // The aperture is a single pixel surrounded by a non-resizing 2 pixel border.
+  // The stroke is actually only 1 pixel wide but we need to use 2 pixels so
+  // that the center patch doesn't try to blend colors between patches.
+  return gfx::Rect(2, 2, 1, 1);

[Evan Stade, 2017/03/01 16:45:49]

ditto

[bokan, 2017/03/01 17:02:31]

Done.

+}
+
 }  // namespace ui