With dynamic vertex conversion the GPU, we will have different input
layouts resulting in different executables. This patch adds a way
of mapping the input layouts to vertex executables.

BUG=angle:560
Change-Id: Ie36f2f8ac2dfcb96f562af577d31f57d6d89b447
Reviewed-on: https://chromium-review.googlesource.com/185192Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>