[WebGPU] Add helper methods for streaming indices in syncClientArrays Bug: angleproject:401499251 Change-Id: I2cab62524ce92a01117282726a3d4de1c266f818 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6438259 Reviewed-by: Matthew Denton <mpdenton@chromium.org> Commit-Queue: Liza Burakova <liza@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>