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: 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}

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
 }