smbprovider: Add MountConfigProto to directory_entry

- Added MountConfigProto since MountConfig options will be passed from chrome-side.

BUG=chromium:888096
TEST=emerges

Change-Id: Ia5514b53b4070738aeb9ff1b55f212d9b90071ec
Reviewed-on: https://chromium-review.googlesource.com/1239199
Commit-Ready: jimmy gong <jimmyxgong@chromium.org>
Tested-by: jimmy gong <jimmyxgong@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>