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This reverts commit c8d093ac.

Reason for revert: Test failures on ChromeOS https://ci.chromium.org/p/chrome/builders/ci/linux-chromeos-chrome/7829

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}

TBR=xiyuan@chromium.org,xiaohuic@chromium.org,wutao@chromium.org

Change-Id: Ic364ef7c1f701b7a36b6503e8d330995de5dcecb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: b/148692733
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363508Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799491}