Revert "Disable use_dawn on Mac for the branch point"

This reverts commit 62e2574d.

Reason for revert: M-75 has been branched.

Original change's description:
> Disable use_dawn on Mac for the branch point
>
> BUG=chromium:852089
> TBR=kbr@chromium.org
>
> Change-Id: I63ba202ac4eb937088ee471a84d8d49ef0a2334a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1571690
> Reviewed-by: Corentin Wallez <cwallez@chromium.org>
> Commit-Queue: Corentin Wallez <cwallez@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#651886}

TBR=kbr@chromium.org

Bug: chromium:852089
Change-Id: I7b4ff29d6240903cc305a849edcb2c5629d3e484
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1576260Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652820}

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
 }