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Note that this feature is being added to read any persisted shares,
however support to write shares as persisted will not be added
until the management UI where shares can be removed is completed.

* prefs: crostini.shared_paths will be a list of filesystem path strings.
* New function CrostiniSharePath::GetSharedPaths() reads from prefs.
* New fileManagerPrivate.getCrostiniSharedPaths executes
  callback to receive list of Entry.
* FileManager.setupCrostini_ loads persisted shares at startup.
* FileManagerApiTest.Crostini verifies reading from prefs and correct
  conversion of filesystem paths to FileEntry.
* New integration test for fileManagerPrivate.getCrostiniSharedPaths().


Bug: 878324
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I5c8ee8193eb2fda94e5505d79b449797fbab9449
Reviewed-on: https://chromium-review.googlesource.com/1220411Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592208}