Tracing: Migrated away from BrowserThread::FILE to TaskScheduler

Bug: 667892
Change-Id: I578c3b3386ee018b8a4ac2a097e56555a34c9f51
Reviewed-on: https://chromium-review.googlesource.com/611188
Reviewed-by: Siddhartha S <ssid@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501498}
5 files changed