Make desktop overlay scrollbars bigger and less transparent.

This makes them easier to see and grab.

BUG=274010

Review URL: https://chromiumcodereview.appspot.com/23658050

git-svn-id: svn://svn.chromium.org/blink/trunk@157845 bbb929c8-8fbe-4397-9dbb-9b2b20218538