The CL changes DataOffer so that it can send file list to the container as drop
data. Before sending a file list to container, we need path conversion because
we don't expose the native file system to containers directly and we mount a
specific directory in the container namespace instead.

To provide the conversion mechnism, the CL defines new class
exo::FileSystemManager.  Because the path conversion logic in chrome/browser
directory, the instance of exo::FileSystemManager is provided by ExoParts,
which is similar to how NotificationSurfaceManager is passed to exo
componenets.

Bug: 750997,b:31988797
Test: To be added
Change-Id: Ic422c1222de24b28fdedfc1833823bf5b20bc737
Reviewed-on: https://chromium-review.googlesource.com/609566


Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Commit-Queue: Daichi Hirono <hirono@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495858}