Oilpan: Add TRACE_EVENT0 to ThreadState::waitUntilSweepersDone.

This event indicates that parallelized sweeping doesn't work well.

BUG=420515
TEST=None; No user-visible behavior changes.

Review URL: https://codereview.chromium.org/672033003

git-svn-id: svn://svn.chromium.org/blink/trunk@184234 bbb929c8-8fbe-4397-9dbb-9b2b20218538