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

This reverts commit b4701e2974a797121a8e553b29c52aa6844bcb3a (Blink revision r160720), AND fixes a bug in the original patch.

The change had initially been reverted because main thread scrolling reasons were not updated
in response to changes to FrameView scrollability. The patch has been updated to handle such
changes.

https://codereview.chromium.org/40373002

BUG=None
R=ajuma@chromium.org, hartmanng@chromium.org

Review URL: https://codereview.chromium.org/66503008

git-svn-id: svn://svn.chromium.org/blink/trunk@161736 bbb929c8-8fbe-4397-9dbb-9b2b20218538