Some block field types, such as nested structs, were bugged. This
only affects our "CollectVariables" path, not our current HLSL
UBO path.

BUG=angle:466

Change-Id: I2b8daf58aa7ec1ad06a80d38f57e76087eacccdc
Reviewed-on: https://chromium-review.googlesource.com/213503Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>