Move extent and dimension conversions to wgpu_utils. This change moves some helper methods out of the ImageHelper and wgpu_helpers and into wgpu_utils. Bug: angleproject:8457 Change-Id: I1cc641e4a48eec30519e331b6150e2432d3bc3c4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5392380 Reviewed-by: Matthew Denton <mpdenton@chromium.org> Commit-Queue: Liza Burakova <liza@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>