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

ui/native_theme/native_theme_aura.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/native_theme/native_theme_aura.h"
 
 #include <limits>
 #include <utility>
 
 #include "base/logging.h"
@@ skipping 17 matching lines @@
 
 namespace ui {
 
 namespace {
 
 // Constants for painting overlay scrollbars. Other properties needed outside
 // this painting code are defined in overlay_scrollbar_constants_aura.h.
 constexpr int kOverlayScrollbarStrokeWidth = 1;
 constexpr int kOverlayScrollbarMinimumLength = 12;
 
+// 2 pixel border with 1 pixel center patch. The border is 2 pixels despite the
+// stroke width being 1 so that the inner pixel can match the center tile
+// color. This prevents color interpolation between the patches.
+constexpr int kOverlayScrollbarBorderPatchWidth = 2;
+constexpr int kOverlayScrollbarCenterPatchSize = 1;
+
 const SkColor kTrackColor = SkColorSetRGB(0xF1, 0xF1, 0xF1);
 
 }  // namespace
 
 ////////////////////////////////////////////////////////////////////////////////
 // NativeTheme:
 
 // static
 NativeTheme* NativeTheme::GetInstanceForWeb() {
   return NativeThemeAura::web_instance();
@@ skipping 262 matching lines @@
         // TODO(bokan): We should probably make sure code using overlay
         // scrollbars isn't asking for part sizes that don't exist.
         // crbug.com/657159.
         break;
     }
   }
 
   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));
+
+  return gfx::Size(
+      kOverlayScrollbarBorderPatchWidth * 2 + kOverlayScrollbarCenterPatchSize,
+      kOverlayScrollbarBorderPatchWidth * 2 + kOverlayScrollbarCenterPatchSize);
+}
+
+gfx::Rect NativeThemeAura::GetNinePatchAperture(Part part) const {
+  DCHECK(SupportsNinePatch(part));
+
+  return gfx::Rect(
+      kOverlayScrollbarBorderPatchWidth, kOverlayScrollbarBorderPatchWidth,
+      kOverlayScrollbarCenterPatchSize, kOverlayScrollbarCenterPatchSize);
+}
+
 }  // namespace ui