[oilpan]: Don't enter safepoint in SafePointAwareMutexLocker if sweeping.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/367633002

git-svn-id: svn://svn.chromium.org/blink/trunk@177290 bbb929c8-8fbe-4397-9dbb-9b2b20218538