Also refactor our current Impl headers slightly to facilitate
parsing by the generation script.

BUG=angle:905

Change-Id: Ib86cff71d18e7f911cf526b27b8a82757bacd738
Reviewed-on: https://chromium-review.googlesource.com/245497Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>