WebGPU: Pack render pass descriptors The render pass descriptors stored in FramebufferWgpu contained references to textures that were fragile and depended on the RenderTargetCache keeping the texture views alive. Refactor the desc so that it does ref counting like the cpp interface and clean up some FramebufferWgpu logic around copying descs. Bug: angleproject:414831373 Change-Id: Ibea96e013474cc43aa30909b596b8898ebccef98 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6546256 Reviewed-by: Liza Burakova <liza@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Matthew Denton <mpdenton@chromium.org>