Ideally this would use the same flags megjablon already set up for other previews.

I am using an enabled/disable type flag. tbansal@ is adding a NQE dropdown flag in about:flags. The user would enable offline previews with default field trial values and only get offline previews on 2G or worse. To simulate slow networks they should use the NQE flag.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/28839dee4819459127f8036e99f32d917601ee50

commit 28839dee4819459127f8036e99f32d917601ee50
Author: ryansturm <ryansturm@chromium.org>
Date: Fri May 05 17:34:27 2017

Adding and Android about:flags feature for offline previews

This adds a command line and about:flags feature for offline previews on
Android.

BUG= 714759 

Review-Url: https://codereview.chromium.org/2854893003
Cr-Commit-Position: refs/heads/master@{#469702}

[modify] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/chrome/browser/about_flags.cc
[modify] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/chrome/browser/flag_descriptions.cc
[modify] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/chrome/browser/flag_descriptions.h
[modify] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/components/previews/core/BUILD.gn
[modify] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/components/previews/core/previews_experiments.cc
[modify] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/components/previews/core/previews_experiments.h
[modify] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/components/previews/core/previews_experiments_unittest.cc
[add] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/components/previews/core/previews_features.cc
[add] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/components/previews/core/previews_features.h
[modify] https://crrev.com/28839dee4819459127f8036e99f32d917601ee50/tools/metrics/histograms/enums.xml