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Restrict the ambient mode flag to only Chromium builds and the Canary/Dev
channels.

Bug: b/148692733
Test: manual
Change-Id: I2554d6d1d07b57eafc079722e188cf079031d3ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2361284
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799329}