webgpu: disconnect dawn_platform before destroying WebGPUDecoderImpl

This patch adds a call of dawn_instance_->DisconnectDawnPlatform() in
WebGPUDecoderImpl::Destroy() so that dawn_instance_->mPlatform can point
to a valid object even when WebGPUDecoderImpl->dawn_platform_ is
destroyed.

Bug: dawn:2349
Change-Id: Ifc8d9e71354a31de5edf8f46b332857b551cdcee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406992


Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280502}