[fsp] Add support for observing entries and notifying about changes.
This patch adds:
1. Flag supports_notify_tag to chrome.fsp.mount().
2. Method chrome.fsp.Notify() to notify about changes of observed entries.
3. Events chrome.fsp.OnObserveDirectory and OnUnobserveEntry.
Missing things, to be added in a separate patch:
1. Glueing chrome.fsp.Notify to the C++ layer properly, not with fake data.
2. Saving and restoring observed entries together with tags from preferences.
TEST=unit_test: *FileSystemProviderProvidedFileSystem*,
*FileSystemProvider*Observe*, *FileSystemProvider*Unobserve*
BUG=248427
Review URL: https://codereview.chromium.org/625463002
Cr-Commit-Position: refs/heads/master@{#299058}
49 files changed
