Use a set instead of a map to keep track of scrollbars.

When keeping track of scrollbars on mac, use a set since
we're not mapping any data.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/23593008

git-svn-id: svn://svn.chromium.org/blink/trunk@157534 bbb929c8-8fbe-4397-9dbb-9b2b20218538