Update comments about scrollbar thickness.

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 ec03635f57af4bed02a460181197917342f0556c..9036cd211c60ce237be323a8fb6cae8f09117c0f 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
@@ -59,6 +59,8 @@ class PLATFORM_EXPORT ScrollbarTheme {
   virtual ScrollbarPart hitTest(const ScrollbarThemeClient&, const IntPoint&);
 
   // This returns a fixed value regardless of device-scale-factor.
+  // This returns thickness when scrollbar is painted.  i.e. It's not 0 even in
+  // overlay scrollbar mode.
   // See also Scrollbar::scrollbarThickness().
   virtual int scrollbarThickness(ScrollbarControlSize = RegularScrollbar) {
     return 0;