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- Add pak_base_dir option to map generated files based on a root
directory other than the directory where the pak file is located.
This is the case for the os settings build, where the pak file is
under gen/c/b/r/settings/chromeos, but paths should be based on
gen/c/b/r/settings.
- Add exclude option to exclude certain files from being unpacked.
This is to allow UIs like the NTP, downloads, and OS settings to
keep files that should not be created by unpack_pak in their grd files.
- Add an assert, ensure unpack_pak.py never creates files/directories
outside of the unpak folder in future. Creating these directories can
cause post-CQ build failures due to race conditions, as seen in the
linked bug.

Bug: 1093405
Change-Id: If85ea9f08c4dbe008ec16ca0a457390012833c56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2251050Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780364}