Redesign HLSL scoped structures to a unique ID. A unique ID gives a more flexible renaming scheme than our current method of using nested scope identifiers. The reduced complexity allows for fewer points of breakage and fixes an outstanding bug with scoped structures (with added test). BUG=angle:618 Change-Id: I6551248bb9fa2d185ab67248721f898dd50151f0 Reviewed-on: https://chromium-review.googlesource.com/202183 Reviewed-by: Nicolas Capens <nicolascapens@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>