Add support for EGL_EXT_client_extensions. BUG=angle:490 Change-Id: Iafc476cef7a3d11b3bb3640e673c28c6df3cf2a1 Reviewed-on: https://chromium-review.googlesource.com/204566 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>