The shader parser used to accept structure definitions in place of
constructors, which is invalid GLSL. This patch fixes that.

BUG=angleproject:939
TEST=angle_unittests

Change-Id: Ibcf502160e91c19e693e9427b548a399d83e2a71
Reviewed-on: https://chromium-review.googlesource.com/312032Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>