This patch adds the structure field member support in SSBO. To support it,
below things are done:
1. Calculate the offset and arrayStride in SSBO structure.
2. Support array of arrays translation.

Bug: angleproject:1951

Change-Id: If8f233e6388d5bb905e78c39a85112d394298138
Reviewed-on: https://chromium-review.googlesource.com/c/1278097
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>