This bug is caught because we haven't implemented
caps.shaderStorageBufferOffsetAlignment on D3D backend. So the default
value is 0. This change will set its value for D3D11 backend.

BUG=angleproject:2625
TEST=angle_deqp_gles31_tests

Change-Id: I03c045c08903e4da41659133e7dbd9c4133186cc
Reviewed-on: https://chromium-review.googlesource.com/1088192Reviewed-by: Yunchao He <yunchao.he@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>