Make sure that the AST root is always a sequence node This enables inserting helper functions as an AST transformation. BUG=angleproject:1116 TEST=angle_unittests Change-Id: I169d4d3a726d0e389cb3444fe9dfb4c6c5d80155 Reviewed-on: https://chromium-review.googlesource.com/290513 Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org>