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This patch implements the destructor of Blink::GPUDevice so that when it
is destroyed in Blink, all the related data structures in both the
client and service side of WebGPU command buffer can also be destroyed.

BUG=chromium:996713

Change-Id: Ibe11aabf395d2dfa33ae3ccfa404d250ffd4c10d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088690Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Cr-Commit-Position: refs/heads/master@{#748102}