Overlay scrollbars flush with window edge

ui/native_theme/native_theme.h

Index: ui/native_theme/native_theme.h
diff --git a/ui/native_theme/native_theme.h b/ui/native_theme/native_theme.h
index 7aa469a50b537f2d293d79f5cbd92bfd7f5c76f8..3933cb4ab135a0c0f82c0c9f448d892a5198233d 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -202,6 +202,10 @@ class NATIVE_THEME_EXPORT NativeTheme {
   struct ScrollbarThumbExtraParams {
     bool is_hovering;
     ScrollbarOverlayColorTheme scrollbar_theme;
+    bool isVerticalScrollbar;

[bokan, 2017/03/22 20:07:25]

You shouldn't need to pass this param from Blink, PaintScrollbarThumb is called
form NativeThemeBase::paint with either kScrollbarHorizontalThumb or
kScrollbarVerticalThumb so you can decide there.

+    bool isLeftVerticalScrollbar;
+    bool hitStart;

[bokan, 2017/03/22 20:07:25]

We should avoid repainting. A simpler solution would be to expand the minimum
and maximum positions of the track by 1 pixel. i.e. the track should start at
-1. Have you tried that?

It looks like we already have something like this in the scrollbar "margin"
though currently in Blink that appears to be used for both dimensions, you'd
need to split it up into separate margins for scroll direction and
perpendicular. On the compositor that gets plumbed through as TrackStart() which
seems like what we're looking for, you'd just need to set a -1 margin. 

+    bool hitEnd;
   };
 
   struct SliderExtraParams {