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This patch implements the storage::WatcherManager interface, which simply calls
the Add/RemoveWatcher methods on ProvidedFileSystemInterface implementations.

An additional callback has been added to AddWatcher, so it's easy to notify
WatcherManager about changes. Without that, we would have to observe provided
file system objects and store callbacks passed to WatcherManager::AddWatcher,
which would make the code complex.

Along the way the storage::WatcherManager interface has been refactored to have
consistent naming with FSP watching methods.

TEST=unit_tests: *ProvidedFileSystem*
BUG=248427

Review URL: https://codereview.chromium.org/689603002

Cr-Commit-Position: refs/heads/master@{#302229}