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

third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h

Index: third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
index a7a5f702f1c91f91158936d7ad438c222d25741b..0233a0abe9294010ab24e22b1cbd1beca4ad6056 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
@@ -185,6 +185,24 @@ class PLATFORM_EXPORT ScrollbarTheme {
 
   virtual bool isMockTheme() const { return false; }
 
+  virtual bool usesNinePatchThumbResource() const { return false; }
+
+  // For a nine-patch scrollbar, this defines the painting canvas size which the
+  // painting code will use to paint the scrollbar into. The actual scrollbar
+  // dimensions will be ignored for purposes of painting since the resource can
+  // be then resized without a repaint.
+  virtual IntSize ninePatchThumbCanvasSize(const ScrollbarThemeClient&) const {
+    NOTREACHED();
+    return IntSize();
+  }
+
+  // For a nine-patch resource, the aperture defines the size of the center

[jbroman, 2017/03/01 15:27:18]

Just the size? I'd expect it to define the center patch (both its size and its
position).

[bokan, 2017/03/01 16:22:44]

You're right, this was an error in the comment. Fixed.

+  // patch that will be stretched out.
+  virtual IntRect ninePatchThumbAperture(const ScrollbarThemeClient&) const {
+    NOTREACHED();
+    return IntRect();
+  }
+
   static ScrollbarTheme& theme();
 
   static void setMockScrollbarsEnabled(bool flag);