Refactors code out of generated functions into a common function and
simplifies assertions.

Bug: angleproject:2958
Change-Id: I896c2304c3ac1e043c16f9ecf81fa8b72a6b87c0
Reviewed-on: https://chromium-review.googlesource.com/c/1355501
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>