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

The reasons for forcing main frame scrolling are applicable for main frame
scrolling. If we are not scrolling the main frame (and scrolling cannot bubble
to the main frame), we should ignore this hint.

R=aelias@chromium.org,jamesr@chromium.org
BUG=254435

Review URL: https://chromiumcodereview.appspot.com/22694003

git-svn-id: svn://svn.chromium.org/blink/trunk@156727 bbb929c8-8fbe-4397-9dbb-9b2b20218538