gpu: Evict invalid Dawn DXGI external images

After https://crrev.com/c/3752055, Dawn DXGI external images are keyed
on the WGPUDevice that created them. This CL keeps the external image
map from growing unbounded by evicting invalid images based on IsValid()
method added in https://dawn-review.googlesource.com/c/dawn/+/95860

Bug: dawn:576, chromium:1338470
Change-Id: Ic481a1a91dd4fba7792bccf3a4f7559779aa098a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3759543
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1023851}