Invisible fixed-position elements should not trigger main thread scrolling

There no reason to scroll on the main thread if the only non-composited fixed
position elements are invisible.

R=vollick@chromium.org
BUG=404332

Review URL: https://codereview.chromium.org/481223002

git-svn-id: svn://svn.chromium.org/blink/trunk@180511 bbb929c8-8fbe-4397-9dbb-9b2b20218538