Move Task Scheduler Initialization From chrome/browser to Content

Content appears to actually be the owner of the process running a
browser. As such, it stands to reason that it should be the one
initializing and controlling the lifetime of the task scheduler.

BUG=662052

Review-Url: https://codereview.chromium.org/2539263003
Cr-Commit-Position: refs/heads/master@{#437452}
10 files changed