This eliminates another global std::map, and replaces it with a json
generated switch. This should be better for threading, better on
memory use, and faster.

BUG=angleproject:1389
BUG=angleproject:1459

Change-Id: I1d289637c00783690ec8ea743ea2aa17b0ab8e50
Reviewed-on: https://chromium-review.googlesource.com/394234Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>