Split the main logic of ThreadState::performPendingSweep() into ThreadState::performPendingSweepInParallel()

ThreadState::performPendingSweep() is a bit too large.
To make it easier to follow what ThreadState::performPendingSweep() does,
this CL splits the main logic of ThreadState::performPendingSweep()
into ThreadState::performPendingSweepInParallel().

No change in behavior.

BUG=420515

Review URL: https://codereview.chromium.org/628093002

git-svn-id: svn://svn.chromium.org/blink/trunk@183256 bbb929c8-8fbe-4397-9dbb-9b2b20218538