[fsp] Allow to create multiple observers for a directory, up to one per origin.
This patch allows to create multiple subscribers for observed entry change
notifications, up to one per origin for the same entry. Eg. multiple extensions
may want to observe the same directory.
Also, the persistent argument is introduced. Persistent subscriber mean, that the
origin is watching the entry persistently, despite reboots. It will be used by
cache only.
Extensions should never use persistent origins. Such subscribers will be removed
on reboot.
TEST=unit_tests: *FileSystemProvider*Registry*,
*FileSystemProvider*ProvidedFileSystem*
BUG=248427
Review URL: https://codereview.chromium.org/642023003
Cr-Commit-Position: refs/heads/master@{#301586}
11 files changed
