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Used for automating ambient mode tast tests.
Add prefs for configuring lock screen inactivity timeout
and background lock screen timeout after ambient mode starts.

BUG=b:169579102

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: I2040ea5a0519b985ad246132925815d87754f7e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485754
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822714}