Use prod Android Messages URL instead of staging.

This change adds a new flag which, if true, points Android Messages
ChromeOS integration to the prod Messages for Web endpoint rather than
the sandbox URL which is being used for staging. This flag is off by
default to continue to allow dogfood until all our web experiments have
rolled out to production.

Bug: 881664
Change-Id: I4e84a19d2a9fb84972889e42644f6fe8d4464389
Reviewed-on: https://chromium-review.googlesource.com/1222880
Commit-Queue: Jeremy Klein <[email protected]>
Reviewed-by: Jeremy Klein <[email protected]>
Reviewed-by: Steven Bennetts <[email protected]>
Cr-Commit-Position: refs/heads/master@{#591172}
8 files changed