Move FrameLoader completion check timer to loading task runner.

WebFrameScheduler is also no longer lazily created: this avoids
problems with the frame scheduler being unexpectedly recreated
during frame detach, which often leads to use-after-frees.

BUG=624694

Review-Url: https://codereview.chromium.org/2172153002
Cr-Commit-Position: refs/heads/master@{#407767}
4 files changed