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This patch adds a EntryWatcherService, which is a bridge between extensions and
fileapi. Also, WatcherManager interface has been created to let backends
implement their own watching logic.

Note, that EntryWatcherService is not wired up to File System API yet. Also,
a lot of features are missing.

TBR=noamsml, jcivelli
TEST=unit_tests: *EntryWatcherService*
BUG=261491

Review URL: https://codereview.chromium.org/452043003

Cr-Commit-Position: refs/heads/master@{#292327}