WebGPU: Avoid poping error scopes too many times The ErrorScope RAII wrapper would pop the webgpu error scope multiple times if the explicit PopScope funciton was called before the destructor. Bug: angleproject:362191737 Change-Id: I8caa7ab39addf03bc09a7cce517cde800c91653c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5815568 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Matthew Denton <mpdenton@chromium.org>