Refactor glVertexAttribIPointer validation. BUG=angleproject:1523 BUG=chromium:668223 Change-Id: I51eca98ad14d8be6f5009149ed6c0daecf569178 Reviewed-on: https://chromium-review.googlesource.com/458106 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>