Only allow overly scrollbars experiement on ChromeOS.

It'll be a while before Aura (Win/Linux) are ready for this.

BUG=274010
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/24281008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225015 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225015