D3D: Make computeOffset not use ErrorOrResult. Unblocks further error refactoring. Also cleans up some checked math logic. Bug: angleproject:2738 Change-Id: I0c9fba9bb908dfc8424921d0db9871241b86e7de Reviewed-on: https://chromium-review.googlesource.com/1142954 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>