Do not take a MessageLoopForIO in the constructor of TaskTrackerPosix.

The MessageLoopForIO isn't available until the service thread
is started. TaskTrackerPosix needs to be instantiated before
that so that SchedulerWorkerPools can be instantiated with a
pointer to it before the TaskScheduler is started.

BUG=690706

Review-Url: https://codereview.chromium.org/2816653003
Cr-Commit-Position: refs/heads/master@{#465586}
5 files changed