CrOS FilesApp: Make persist optional for crostini path sharing

Only persist shares from right-click > Share with Linux.
Do not persist shares that are done for the purpose of
opening a file with a linux app.

Share with Linux is only supported for directories (not files),
but open with linux app will share only the selected files, and
not the containing directories.

Also needed to fix chrome.fileManagerPrivate.executeTask jsdoc
to correctly declare that callback receives
!chrome.fileManagerPrivate.TaskResult.  I'm not sure why closure
compiled previously without this fix.

Bug: 878324
Change-Id: Idfe0728cd817ba406b0dd1a1eb80c561944f1ce6
Reviewed-on: https://chromium-review.googlesource.com/c/1290581
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602256}
19 files changed