Oilpan: Fix threading races on ThreadState::s_mainThreadStackStart

I forgot to add a isMainThread() check to ~ThreadState(). This CL just adds the check.

BUG=427657

Review URL: https://codereview.chromium.org/640353004

git-svn-id: svn://svn.chromium.org/blink/trunk@184492 bbb929c8-8fbe-4397-9dbb-9b2b20218538