Micro-optimize State::hasMappedBuffer. This speeds up draw call validation. BUG=angleproject:959 Change-Id: I8272e36201521f158823739a6604444fb5e66b6e Reviewed-on: https://chromium-review.googlesource.com/266025 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>