Make scheme sets in SchemeRegistry thread-safe
Make scheme sets in SchemeRegistry thread-safe

The scheme sets are shared across threads, and they seem to be touched even
after new threads are created, so it looks we probably need a global lock.

BUG=459515

Review URL: https://codereview.chromium.org/941313002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@190576 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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