Revert "Disable use_dawn for the branch point."

This reverts commit 6556b2ce4afd91f9a24486c1241e266e6eb124e3.

Reason for revert: Branch point has passed.

Original change's description:
> Disable use_dawn for the branch point.
> 
> 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: I979ae6c24087d4037d57ce95ea33f119c604e91c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715271
> Reviewed-by: Corentin Wallez <cwallez@chromium.org>
> Commit-Queue: Corentin Wallez <cwallez@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#680395}

TBR=kbr@chromium.org,cwallez@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromium:985142
Change-Id: Ia8a260e7d02e4a3326befb9c526d689a2c156271
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722012
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681563}