Revert "Remove all remaining deprecated Web/BrowserThreads!!"

This reverts commit 6127a86f3a3b0d4096b3c616d379e088effe8170.

Reason for revert: Suspect this caused spike in ThreadWatcher hang reports: crbug.com/768886

Original change's description:
> 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}

TBR=gab@chromium.org,jam@chromium.org,dpranke@chromium.org,asvitkine@chromium.org,sdefresne@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

NOPRESUBMIT=True (for restoring the deprecated browser threads)

Bug: 689520, 365909, 752144, 768886
Change-Id: I53d93572118303743e1dde71b2a473350717b215
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
Reviewed-on: https://chromium-review.googlesource.com/689254
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504939}
27 files changed