This should improve ANGLE startup time by avoiding creating a large
std::map filled with the entry points.

BUG=chromium:781460

Change-Id: I20cfdb10b99844d0f60759dda73b729991dc60fe
Reviewed-on: https://chromium-review.googlesource.com/768209Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>