Atomic: Notify Watcher to observe device fd
Atomic: Notify Watcher to observe device fd

we always block on the main thread till the FD becomes
readable. With non-atomic code path, we always wait on a
worker thread to see if the device fd has become readable.
We want to do the same with Atomic, notify watcher to observe
if the device fd is readable in a separate thread (respecting is_sync value)
instead of blocking main thread.

BUG=

Review URL: https://codereview.chromium.org/1282723002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#342651}

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