Fixed scrolling using the mouse wheel inside regions.

When searching for the scroll ancestor and encountering a named flow thread,
the search will continue with the DOM ancestor of the top-most element
in the named flow thread.

BUG=315479

Review URL: https://codereview.chromium.org/84923005

git-svn-id: svn://svn.chromium.org/blink/trunk@163278 bbb929c8-8fbe-4397-9dbb-9b2b20218538