formatutils: allow reusing rowPitch computation for depthPitch This should fix a null D3D11 backend draw call performance regression. BUG=651101 Change-Id: I2eb10cddd15f0e7b25b886c89eccd2906e988c72 Reviewed-on: https://chromium-review.googlesource.com/392227 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>