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This change lets us observe screen idle state (on/off state) so that
we can invalidate send and receive surfaces in response.

This also collapses PowerClientChromeos into PowerClient since we no
longer need to guard Chrome OS-specific functionality.

Fixed: 1169262
Change-Id: I893df57d113590c0e09a81a381c3a291b74fd00a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643918Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/master@{#846278}