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: refs/heads/master@{#342651}