Create TaskScheduler in WebMainLoop constructor.

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

This change matches a BrowserMainLoop change:
https://codereview.chromium.org/2835933004/

BUG=690706

Review-Url: https://codereview.chromium.org/2841943005
Cr-Commit-Position: refs/heads/master@{#467439}
1 file changed