More entry point validation refactor for auto-gen.

BUG=angleproject:747

Change-Id: I9462a28838df3f265e1401f66c838cc5a73d511f
Reviewed-on: https://chromium-review.googlesource.com/636517Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>