Fix mac scrollbar thumb width with --enable-use-zoom-for-dsf

Bug: 716231
Change-Id: I20e23ea9c6f908d188a993f40cd036715b09748f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2360420Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799127}

third_party/blink/renderer/core/scroll/scrollbar_theme_mac.mm

@@ -374,7 +374,7 @@ void ScrollbarThemeMac::PaintThumbInternal(GraphicsContext& context,
         setBoundsSize:NSSizeFromCGSize(CGSize(scrollbar.FrameRect().Size()))];
     [observer setSuppressSetScrollbarsHidden:NO];
 
-    thumb_size = [scrollbar_painter trackBoxWidth];
+    thumb_size = [scrollbar_painter trackBoxWidth] * scrollbar.ScaleFromDIP();
   }
 
   if (!scrollbar.Enabled())