Enable use_dawn by default on Mac

Mac is the first platform where WebGPU is implemented. Enabling use_dawn
there makes sure that dawn-related tests are run on the CQ, and allows
developers to use Mac Canary to test WebGPU.

BUG=chromium:852089

Change-Id: Ic076afbe4d6d9efe97bee7bb56f0d375a35bec5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559319Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649264}

ui/gl/features.gni

@@ -12,7 +12,7 @@ declare_args() {
   use_egl = is_win || is_android || is_linux || is_fuchsia || is_mac
 
   # Should Dawn support be compiled to back the WebGPU implementation.
-  # Also controls linking Dawn depdencies in such as shaderc, SPIRV-Tools and
+  # Also controls linking Dawn depedencies in such as SPIRV-Tools and
   # SPIRV-Cross
-  use_dawn = false
+  use_dawn = is_mac
 }