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..208f06ac89d24125baab3d6a12cfaf41d8e068ee 100644
--- a/ui/native_theme/native_theme_aura.cc
+++ b/ui/native_theme/native_theme_aura.cc
@@ -34,6 +34,8 @@ namespace {
 // this painting code are defined in overlay_scrollbar_constants_aura.h.
 constexpr int kOverlayScrollbarStrokeWidth = 1;
 constexpr int kOverlayScrollbarMinimumLength = 12;
+const gfx::Size kOverlayScrollbarNinePatchCanvasSize(5, 5);

[Evan Stade, 2017/03/02 02:32:01]

sorry, what I actually meant here was something like

constexpr int kBorderStroke = 2;
constexpr int kCenterPatchSize = 1;

then down in the functions below

return gfx::Size(kBorderStroke * 2 + kCenterPatchSize, kBorderStroke * 2 +
kCenterPatchSize);

return gfx::Rect(kBorderStroke, kBorderStroke, kCenterPatchSize,
kCenterPatchSize);

[bokan, 2017/03/02 13:55:36]

Ah, that does sound better. Done (names slightly altered).

+const gfx::Rect kOverlayScrollbarNinePatchAperture(2, 2, 1, 1);
 
 const SkColor kTrackColor = SkColorSetRGB(0xF1, 0xF1, 0xF1);
 
@@ -317,4 +319,29 @@ 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. The border is 2 pixels so that
+  // the inner pixel can match the center tile color. This prevents color
+  // interpolation between the patches.
+  return kOverlayScrollbarNinePatchCanvasSize;
+}
+
+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 kOverlayScrollbarNinePatchAperture;
+}
+
 }  // namespace ui