The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f6523727f2df581e2a825316636909e2afdb1a4f

commit f6523727f2df581e2a825316636909e2afdb1a4f
Author: Gregory Chatzinoff <gchatz@chromium.org>
Date: Tue Nov 21 01:33:53 2017

Use VariationsService::SetupFieldTrials in iOS.

iOS currently has a forked version of
VariationsService::SetupFieldTrials. This CL refactors IOSMainParts to
use the version from VariationsService instead, since it is better
maintained. The CL also removes duplicated iOS switches and creates
an iOS version of the PlatformFieldTrials object, as the
SetupFieldTrials API requires it. Since the prior SetupIOSFieldTrials
API only had an expired, unused experiment, it could be deleted.

Bug:  784996 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I2c24e8435bb6a135e798d521ec899dff5902bd98
Reviewed-on: https://chromium-review.googlesource.com/773080
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: Gregory Chatzinoff <gchatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518036}
[modify] https://crrev.com/f6523727f2df581e2a825316636909e2afdb1a4f/ios/chrome/browser/BUILD.gn
[modify] https://crrev.com/f6523727f2df581e2a825316636909e2afdb1a4f/ios/chrome/browser/about_flags.mm
[modify] https://crrev.com/f6523727f2df581e2a825316636909e2afdb1a4f/ios/chrome/browser/chrome_switches.cc
[modify] https://crrev.com/f6523727f2df581e2a825316636909e2afdb1a4f/ios/chrome/browser/chrome_switches.h
[delete] https://crrev.com/aef1e5e54e5e66fda8ee414336107c3be78162fd/ios/chrome/browser/ios_chrome_field_trials.cc
[modify] https://crrev.com/f6523727f2df581e2a825316636909e2afdb1a4f/ios/chrome/browser/ios_chrome_field_trials.h
[modify] https://crrev.com/f6523727f2df581e2a825316636909e2afdb1a4f/ios/chrome/browser/ios_chrome_main_parts.h
[modify] https://crrev.com/f6523727f2df581e2a825316636909e2afdb1a4f/ios/chrome/browser/ios_chrome_main_parts.mm