Allow enabling GL_EXT_texture_filter_anisotropic. BUG=angleproject:1721 Change-Id: I7cbc734915cde7d09165a3fcfe9a6bc0d7149aff Reviewed-on: https://chromium-review.googlesource.com/445959 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>