Refactor EGL config entry points to to separate validation. BUG=angleproject:1707 BUG=angleproject:798 Change-Id: I34fefa9f9bbd42ef05f222c8caf856a22cf206a3 Reviewed-on: https://chromium-review.googlesource.com/428293 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>