[WebGLOnWebGPU] Fix texture target handling in deleteTexture

1. Add validation for invalid texture targets to avoid OOB access in
   bound_textures_.
2. Fix the binding cleanup loop to ensure all texture units are
   iterated over.

Bug: 466163478
Change-Id: Id386db9245f9d1b9b1990a483269b3e99b1553bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7231049
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1558353}