Sometimes it is necessary to recreate a VS toolchain package with a tiny
change such as editing or removing a single file. Recreating the VS
environment can be time consuming or even impossible (the VS installer
does not allow going to arbitrary versions) so this switch allows
repackaging an existing unpacked directory. The typical usage would be
to make the modifications to one of the toolchain directories in
depot_tools\win_toolchain\vs_files and then repackage it using:

    python package_from_installed.py --repackage third_party\depot_tools\win_toolchain\vs_files\9bc7ccbf9f4bd50d4a3bd185e8ca94ff1618de0b

Bug: 772123
Change-Id: I77b928f695e5f07e33f68dd37711c8761a3c7a22
Reviewed-on: https://chromium-review.googlesource.com/713562Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>