Fix linking of struct varyings. The dEQP varying struct linkage tests were triggering asserts when we tried to query component info of struct types. BUG=angle:580 Change-Id: I61e33573c5577a327c58fec7a07ae7718690ac42 Reviewed-on: https://chromium-review.googlesource.com/189194 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>