This patch is the first one of the series that organize
implementation dependent resource limits on every type
of shader into ShaderMap and clean up all the related
code.

In the next patch all such resource limits are put in
the corresponding ShaderMaps.

BUG=angleproject:2169

Change-Id: I40cb58c55b2e82df33221ddb36eff0abcd7e8b22
Reviewed-on: https://chromium-review.googlesource.com/1034108
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>