Enable always-available extensions in gl::Context. We can consolidate exposing these extensions in initCaps. Otherwise we have to maintain the lists in every Renderer back-end. Also do the same treatment for select egl::Display extensions. BUG=angleproject:1319 Change-Id: I529dd120c6d2cdbb789bd9dd20491e796e97f3f6 Reviewed-on: https://chromium-review.googlesource.com/345914 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>