This allows us to reuse it in our other BUILD.gn files.

Bug: angleproject:3353
Change-Id: Ic88f7781010d7319dfe3bb8f2f942fa08d37e4dc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1614097
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>