Remove WebThreads that aren't used anymore.

The FILE_USER_BLOCKING and CACHE WebThreads are deprecated an aren't
used anymore.

Bug: 689520
Change-Id: I9facb7539513b22cdae0688f088e9bbd10dfb73c
Reviewed-on: https://chromium-review.googlesource.com/595873
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Francois Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491061}
6 files changed