Main frame scroll layer bounds should be updated after layout.

Not sure if this is a complete fix for this bug, but it seems logical
that the main frame scroll layer bounds should be checked, and possibly
updated, after layout.

BUG=328672

Review URL: https://codereview.chromium.org/138963002

git-svn-id: svn://svn.chromium.org/blink/trunk@165333 bbb929c8-8fbe-4397-9dbb-9b2b20218538