This CL introduces adds an argument to SchedulerWorker::Create that
determines whether COM STA should be initialized on threads created by
the SchedulerWorker. Previously, COM STA was always initialized on
SchedulerWorker threads. This caused crashes in renderer processes
(CoInitializeEx may call CreateWindowEx which is banned in
renderer processes).

The argument does nothing on platforms other than Windows.

BUG=681114

Review-Url: https://codereview.chromium.org/2654823002
Cr-Commit-Position: refs/heads/master@{#446071}