GitLab

When a user opens files within Downloads using a crostini app,
show a dialog for the user to give permission to share the
directory that the files are in with crostini.

* Move handling of 'crostin-files' flag checking into
  foreground/js/crostini.js
* Change FileTasks.executeInternal to detect files that
  can be shared with crostini and do sharing before execute.
* UI Test verifies share dialog is shown before execute.
* ConvertFileSystemURLToPathInsideCrostini maps
  path from within Downloads to '/ChromeOS/Downloads/...'

Bug: 878324
Change-Id: Ie88090ff958dd201aeacb23fdfcc84a4f3d20210
Reviewed-on: https://chromium-review.googlesource.com/c/1243871Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596003}