WebGPU: Fix bind groups leaking in CommandBuffer. Wrap all per-submission member variables in a struct so that we can reset everything at once using the default constructor. This avoids similar leaks in the future. Bug: chromium:413078308 Change-Id: I3f926c82905b73b27d6b751cfe2878a41ced916a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6652290 Reviewed-by: Matthew Denton <mpdenton@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>