Test: ShaderStorageBufferTest31.ShaderStorageBufferReadWriteOffset.
Align kBufferOffset to bufferAlignOffset correctly.

Bug: angleproject:4471
Change-Id: I71084839a845138b32d58d8bd2cf6fed2633a9be
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2100047
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>