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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}