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The current code in FileHelper assumes all windows are Arc. We now
check the source and target aura::Window to detect and do the
appropriate path translations and share files when necessary with
crostini/pluginvm.

For example, when crostini is the drag source, we will convert paths
such as:

 <homedir>/file => /media/fuse/crostini_<hash>_termina_penguin/file

And do the opposite translation when crostini is the drop target.

We also convert paths and share files such as:

 <cryptohome>/MyFiles/file => /mnt/chromeos/MyFiles/file
 /media/removable/MyUSB => /mnt/chromeos/removable/MyUSB

For PluginVM, the translation is

 <cryptohome>/MyFiles/file => //ChromeOS/MyFiles/file
 /media/removable/MyUSB => //ChromeOS/removable/MyUSB

Bug: 1144138
Change-Id: I2bb38336537acf13c4797374d6dabe6e518d689e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517346
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Jason Lin <lxj@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827956}