Add a lock to WTF::Partitions::initialize().
Add a lock to WTF::Partitions::initialize().

BUG=270531

Review URL: https://codereview.chromium.org/26942002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@159406 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs