Oilpan: Fix threading races on ThreadState::s_mainThreadStackStart
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@184492 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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