Also rename ImplFactory to GLImplFactory.

This will allow us to use the same factory design pattern for EGL
objects, and to use State helper classes to share data with Impls.

BUG=angleproject:1363
BUG=angleproject:1369

Change-Id: I07a8fe40838d5d4ca32b04910c306edeab4d25a7
Reviewed-on: https://chromium-review.googlesource.com/342051Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>