Fix int-cast overflow in ScrollbarTheme

If we're at an offset that can't be represented at an int then the page
probably isn't usable but it's better to saturate than overflow here.

Bug: 1068200
Change-Id: I07afcf9b0486fac3311c0a128a1932b768b69d5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137827
Auto-Submit: David Bokan <bokan@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757542}

third_party/blink/renderer/core/scroll/scrollbar_theme.cc

@@ -191,7 +191,7 @@ int ScrollbarTheme::ThumbPosition(const Scrollbar& scrollbar,
       return 0;
     float pos = std::max(0.0f, scroll_position) *
                 (TrackLength(scrollbar) - ThumbLength(scrollbar)) / size;
-    return (pos < 1 && pos > 0) ? 1 : pos;
+    return (pos < 1 && pos > 0) ? 1 : base::saturated_cast<int>(pos);
   }
   return 0;
 }