This will be used to set up gn args for standalone builds.
We can then set the GN args source-side instead of in the
recipe directly.

Bug: angleproject:5114
Change-Id: I6fac6d13903951031a7299aea203f6b3e13c72bc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2819461Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>