Revert "Disable use_dawn for the branch point."

This reverts commit ffe8de71.

Reason for revert: Branch point has passed.

Original change's description:
> Reland "Disable use_dawn for the branch point."
>
> Original change's description:
> > Revert "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.
> > >
>
> Bug: chromium:985142
> Change-Id: If23eeb89a3130eafe1def72e293d60c0bd0222e1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867091
> Reviewed-by: Kai Ninomiya <kainino@chromium.org>
> Commit-Queue: Kai Ninomiya <kainino@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#707156}

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

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

Bug: chromium:985142
Change-Id: I913a829a911296f1e1c437a72e8eb976dbefeb89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872640
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707941}

ui/gl/features.gni

@@ -14,5 +14,5 @@ declare_args() {
   # Should Dawn support be compiled to back the WebGPU implementation.
   # Also controls linking Dawn depedencies in such as SPIRV-Tools and
   # SPIRV-Cross
-  use_dawn = false
+  use_dawn = is_mac
 }