Adds export_targets.py to run as part of presubmit in order to help
prevent breaking Firefox with BUILD.gn changes.

Bug: chromium:1003151
Change-Id: I5a7ab00891cd7c094c797e6150f642f803a726b6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1802038
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>