Add EGLImplFactory. 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/342051 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>