Remove WebThread::FILE.

This thread is deprecated and no longer used.

Bug: 689520
Change-Id: I5258d8e0892bb55326ebf6c3f41966f0757cd527
Reviewed-on: https://chromium-review.googlesource.com/657885
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Francois Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501029}
8 files changed