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This is a reland of c8d093ac

Original change's description:
> ambient: Use different backend servers
> 
> This patch configs the dev channel to use testing server. But users can
> overwrite it with a flag.
> 
> Bug: b/148692733
> Test: manual
> Change-Id: I967281bc1421c37d9c1ff74a0815d6a6da570da3
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342134
> Commit-Queue: Tao Wu <wutao@chromium.org>
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#799318}

Bug: b/148692733
Change-Id: Id8140c368da44089e15ac9ded3b4faaf8f19efef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364267Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799793}