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If we drop an unshared file into Plugin VM, we do not share it since
Plugin VM requires the whole directory to be shared for file dances.
Instead, we show the 'Move to Windows files' dialog in FilesApp.

ChromeFileHelper sends new CrostiniEvent
'drop_failed_plugin_vm_directory_not_shared' and FilesApp will
show the dialog if it has a drag in process.

Bug: 1144138
Change-Id: I647978390aa451af7fb23ce28640d3511c96c4b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2556742
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Jason Lin <lxj@google.com>
Cr-Commit-Position: refs/heads/master@{#830880}