The previous code was using a completely unrelated variable to compute
max combined uniform blocks limit.

BUG=angleproject:2099

Change-Id: I9e56b50a92790f525dda50adca52b6ac5edfc95a
Reviewed-on: https://chromium-review.googlesource.com/562276
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>