[//gpu] Require WEBGPU_WRITE when passing WEBGPU_MAILBOX_DISCARD

This CL enforces the requirement that passing WEBGPU_MAILBOX_DISCARD
in AssociateMailbox() requires that the SharedImage being associated has
WEBGPU_WRITE usage. The reason is that passing WEBGPU_MAILBOX_DISCARD is
a request to lazily clear the texture, which is a write operation.

The only callsite that passes WEBGPU_MAILBOX_DISCARD is from the WebGPU
swapchain [1], which indeed creates SharedImages with
WEBGPU_WRITE usage [2].

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/gpu/webgpu_swap_buffer_provider.cc;l=221?q=WEBGPU_MAILBOX_DISCARD&sq=&ss=chromium
[2] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/gpu/webgpu_swap_buffer_provider.cc;l=143-160;drc=41d53a863ef269e6d53fea38cc1d3feda6c3df78;bpv=1;bpt=1

Bug: 339171225
Change-Id: Ieeefbb37f38eb5bcf70c22f9ff4bd29293cb1524
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5526447
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1307325}