ES31: Fix the bug that SSBO is active but not statically used If a SSBO is active but not statically used, there will be no UAV slot is bound. So we should skip this kind of block. Bug: angleproject:1951 Change-Id: I4d813ddefcce6c31fa02701f26148eb21c00f380 Reviewed-on: https://chromium-review.googlesource.com/c/1381847 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>