Our caching of compiled vertex shaders according to their input
signature needs a consistent input signature to match up shaders.
Since the linking step currently shuffles attributes according
to API specified locations we need a separate list.

BUG=angle:560
Change-Id: Icc4d5df1d37ae895d5c882b86e60f05b5c268461
Reviewed-on: https://chromium-review.googlesource.com/185193Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>