The patch refactors the vertex input HLSL to allow for dynamic
conversion in the future. Using a placehold keyword, the code
replaces that stub with the vertex input conversion logic.

BUG=angle:560
Change-Id: I100c50cb4decd45b0f0c10d7c2c22583368b773e
Reviewed-on: https://chromium-review.googlesource.com/185194Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>