Disable use_dawn on Mac

This was meant to be disabled before the branch point but we missed it,
so disabling on top of tree to ask for a merge request for M76.

BUG=chromium:955699
BUG=chromium:852089
TBR=kbr@chromium.org

Change-Id: I055e07b421b38a6ea24d753e5d0b53881f93a2c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643510Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666012}

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 = is_mac
+  use_dawn = false
 }