Restore thinning fade animator for overlay scrollbars on non-Android/Mac.

The correct scrollbar animation action for overlay scrollbars on ChromeOS, Linux & Windows was overwritten by

https://codereview.chromium.org/23983047

This CL restores the correct animator.

BUG=none
R=piman@chromium.org

Review URL: https://codereview.chromium.org/154753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248991 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248991