Partially revert "Ignore should-scroll-on-main-thread if main frame is not scrollable"

This partially reverts Blink r157644
(https://codereview.chromium.org/23641004), which can cause a frame
that needs to scroll on the main thread to incorrectly scroll on the
compositor thread, causing non-composited fixed position elements to
bounce around.

BUG=296823

Review URL: https://codereview.chromium.org/40373002

git-svn-id: svn://svn.chromium.org/blink/trunk@160720 bbb929c8-8fbe-4397-9dbb-9b2b20218538