With a unique ID, similar to how we handle scoped structures
in the HLSL translator, we can simplify the declared structure
check.

BUG=angle:618

Change-Id: I5fe61c8d353650ac67a7c7ecdf53a70b91a9b717
Reviewed-on: https://chromium-review.googlesource.com/202500Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>