This should avoid generating diffs when running the code generation with
different source directories or on different platforms.

BUG=angleproject:2695

Change-Id: I67776883bdbeb867a49bea00f16998c04f7857b4
Reviewed-on: https://chromium-review.googlesource.com/1127355
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>