Processes that need to initialize task scheduler with custom params
and processes that need to create and start task scheduler in separate
steps should use TaskScheduler::Create.

Processes that need a task scheduler but don't need custom
initialization params and are fine with creating and starting the
task scheduler in one step (i.e. don't need a period during which
tasks can be posted to task scheduler without running) should
use TaskScheduler::CreateAndStartWithDefaultParams.

BUG=690706

Review-Url: https://codereview.chromium.org/2836033002
Cr-Commit-Position: refs/heads/master@{#467159}