Don't set WTF::Partitions::s_initialized to true before initializing

In theory, a simultaneous caller to Partitions::getBufferPartition()
could see the flag as being true, and return the uninitialized buffer
partition, while another thread is initializing it.

BUG=367672

Review URL: https://codereview.chromium.org/268673012

git-svn-id: svn://svn.chromium.org/blink/trunk@175020 bbb929c8-8fbe-4397-9dbb-9b2b20218538