Move OomKillsMonitor from a single-threaded SequencedWorkerPool to a non-joinable thread.

The semantics are equivalent after this CL:
 - SequencedWorkerPool::CONTINUE_ON_SHUTDOWN => non-joinable thread + leaky instance.
   - Actually, it's better than before because with lack of a Leaky lazy instance,
     there was a join despite the CONTINUE_ON_SHUTDOWN request [1].
 - Ramping worker thread down on destruction through OomKillsMonitorHandle.

[1] https://cs.chromium.org/chromium/src/base/threading/sequenced_worker_pool.cc?rcl=1469791693&l=601

BUG=622400

Review-Url: https://codereview.chromium.org/2197753002
Cr-Commit-Position: refs/heads/master@{#411899}
4 files changed