Should be both experimental web platform feature and an additional flag. Removing something from experimental web platform features requires an intent to ship email to blink-dev@ , which I'd like to do only when we ship on desktop.

Update: should retain --enable-experimental-web-platform-features and add another feature flag

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/42a50cb83ebc41b44033d1a77ecd3e719981b29d

commit 42a50cb83ebc41b44033d1a77ecd3e719981b29d
Author: mathp <mathp@chromium.org>
Date: Tue Feb 28 12:24:42 2017

[Payments] Put Web Payments feature on desktop behind flag

--enable-features=WebPayments

This is in addition to --enable-experimental-web-features

BUG= 696032 
TEST=builds, all tests

Review-Url: https://codereview.chromium.org/2715013002
Cr-Commit-Position: refs/heads/master@{#453575}

[modify] https://crrev.com/42a50cb83ebc41b44033d1a77ecd3e719981b29d/chrome/app/generated_resources.grd
[modify] https://crrev.com/42a50cb83ebc41b44033d1a77ecd3e719981b29d/chrome/browser/about_flags.cc
[modify] https://crrev.com/42a50cb83ebc41b44033d1a77ecd3e719981b29d/chrome/browser/chrome_content_browser_client.cc
[modify] https://crrev.com/42a50cb83ebc41b44033d1a77ecd3e719981b29d/chrome/browser/payments/site_per_process_payments_browsertest.cc
[modify] https://crrev.com/42a50cb83ebc41b44033d1a77ecd3e719981b29d/chrome/browser/ui/views/payments/payment_request_interactive_uitest_base.cc
[modify] https://crrev.com/42a50cb83ebc41b44033d1a77ecd3e719981b29d/content/public/common/content_features.cc
[modify] https://crrev.com/42a50cb83ebc41b44033d1a77ecd3e719981b29d/content/public/common/content_features.h
[modify] https://crrev.com/42a50cb83ebc41b44033d1a77ecd3e719981b29d/tools/metrics/histograms/histograms.xml