Remove all remaining deprecated Web/BrowserThreads!!

(except BrowserThread::FILE which has a handful use cases left still and
will follow suit shortly)

(BrowserThread::PROCESS_LAUNCHER migration was put on hold because of
Android specific requirements around process launching)

Remaining usage of TestBrowserThreadBundle::REAL_DB_THREAD could simply
be removed as all of the product code was already migrated to TaskScheduler
which always uses real threads in unit tests.

Bug: 689520, 365909, 752144
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
Change-Id: Id70b722aa35e8568795d5aac16947392ac1597be

NOPRESUBMIT=True (for content/browser/browser_thread_impl.cc:211)

Change-Id: Id70b722aa35e8568795d5aac16947392ac1597be
Reviewed-on: https://chromium-review.googlesource.com/610880
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503732}
27 files changed