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This allows users to configure a custom domain through the UI at
chrome://flags allowing for easier development and testing. For
simplicity, this also updates the previous command line flags to use
custom app/install urls to a new flag: --custom-android-messages-domain
which accepts a single domain and appends the correct suffixes in code.

Bug: 1055472
Change-Id: Ie3b1a69c258642fbe7e7f579cbf7f0b221d96f95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154101
Commit-Queue: Jon Mann <jonmann@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Azeem Arshad <azeemarshad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760229}