Document what pieces of data the global `FutexEmulation::mutex_`
mutex protects from concurrent access, and reduce the scope
in which said mutex is locked during `FutexEmulation::Wait()`
to match that description more closely.

Change-Id: I0764efabac06814d83ed5c4af4eb7da34af47cab
Reviewed-on: https://chromium-review.googlesource.com/1074689
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53429}