Suppress commits in single threaded mode during page load

This is a prerequisite for https://codereview.chromium.org/134623005/
and depends on https://codereview.chromium.org/380853002/.

If this is not done with the above patch, then during document closing
the cc::Scheduler will attempt to create commits and layout when it
should not.

R=jamesr@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/376293002

git-svn-id: svn://svn.chromium.org/blink/trunk@177886 bbb929c8-8fbe-4397-9dbb-9b2b20218538