Rename Framebuffer::completeness to checkStatus and add an Impl method. BUG=angle:841 Change-Id: I04b4ffd086424569a15aa21447dd552e0a898928 Reviewed-on: https://chromium-review.googlesource.com/232394 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>