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/267797Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>