Don't bubble scrolls across boundary when scrolling to anchor

Takes the same approach as FrameLoader to ensure that we don't
bubble scrolls incorrectly.

BUG=415141

Review URL: https://codereview.chromium.org/606023003

git-svn-id: svn://svn.chromium.org/blink/trunk@182954 bbb929c8-8fbe-4397-9dbb-9b2b20218538