Migrate the PowerTracingAgent away from using BrowserThread::FILE

This is part of a broader migration (see http://bit.ly/2f8ookS), after
which BrowserThread::FILE will be no more.

Bug: 761496, 667892
Change-Id: I7a6542091a86e6e84b0f83dd799567232473a699
Reviewed-on: https://chromium-review.googlesource.com/665386
Commit-Queue: Charlie Andrews <charliea@chromium.org>
Reviewed-by: Francois Doray <fdoray@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502054}
2 files changed