Make EGL image extensions enableable. BUG=angleproject:1523 Change-Id: I6e890380bafb9f81595ab603996259fe6177e9e0 Reviewed-on: https://chromium-review.googlesource.com/744447 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>