Create TaskScheduler in BrowserMainLoop constructor.

This CL allows tasks to be posted via base/task_scheduler/post_task.h
as soon as BrowserMainLoop is instantiated. Tasks don't run until
TaskScheduler::Start() is called in BrowserMainLoop::CreateThreads().

BUG=690706

Review-Url: https://codereview.chromium.org/2835933004
Cr-Commit-Position: refs/heads/master@{#467283}
1 file changed