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This patch exposes the read-only attribute GPUAdapter.extensions in
Blink as is required in WebGPU SPEC. GPUAdapter.extensions is a
dictionary whose keys are extension names and values are boolean values
that indicate if the extensions are supported on this adapter or not.

BUG=chromium:996713

Change-Id: I6bfd2283babf981208ade10adb458f088f0333f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929717Reviewed-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@{#718409}