WebGPU: Refactor syncClientArrays. This change adds a few helper methods for syncClientArrays to improve readability Bug: angleproject:401499251 Change-Id: Ida427f80e07f469b83ff12850169b2110e65743c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6441946 Reviewed-by: Matthew Denton <mpdenton@chromium.org> Commit-Queue: Liza Burakova <liza@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>