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The responsibility of EntryWatcherService has been moved to WatcherManager
implementations.

Note, that the original code was unnecesarily complicated. After replacing
the observer interface with a simple callback, there is no need for an extra
service managing watchers anymore.

This was possible because with the new design entry watchers will no longer
survive reboots.

TEST=Compiles.
BUG=261491

Review URL: https://codereview.chromium.org/642343004

Cr-Commit-Position: refs/heads/master@{#301319}