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This flag controls the inclusion of Dawn and the availability of WebGPU
which we want to keep limited to Canary for now.

TBR=kbr@chromium.org
BUG=chromium:985142

Change-Id: I7677ec0571bcf394af4442829ad86e92a87b8a6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1784611Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693224}