Revert "Perform layout when adding or removing custom scrollbars"

This reverts r164352 and r164548. The former tried to rebaseline
a reftest, which is not something that makes sense.

TBR=ccameron@chromium.org,adamk@chromium.org

Review URL: https://codereview.chromium.org/126943002

git-svn-id: svn://svn.chromium.org/blink/trunk@164640 bbb929c8-8fbe-4397-9dbb-9b2b20218538