hlsl: Fix struct specifiers in uniforms. We would miss the definition for structs specfied in uniforms. Fix this by always checking to add the constructor. Fixes the WebGL test 'glsl/misc/struct-specifiers-in-uniforms'. BUG=angleproject:818 BUG=433412 Change-Id: I411e4a4477f7ef34fceb9faa77489f77d8efdce8 Reviewed-on: https://chromium-review.googlesource.com/267797 Reviewed-by: Zhenyao Mo <zmo@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>