Revert "Enable use_dawn by default on Mac"

This reverts commit c3314088.

Reason for revert: speculative revert for WebGPUDecoderTest.Dummy failures
- https://ci.chromium.org/p/chromium/builders/ci/Mac10.10%20Tests/41527
- https://ci.chromium.org/p/chromium/builders/ci/Mac10.10%20Tests/41528

Original change's description:
> 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/+/1559319
> Reviewed-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}

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

Change-Id: I125e6f63941fdf0007b42da1673f641a7b32de5d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:852089
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1560530Reviewed-by: Collin Baker <collinbaker@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649325}

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 depedencies in such as SPIRV-Tools and
+  # Also controls linking Dawn depdencies in such as shaderc, SPIRV-Tools and
   # SPIRV-Cross
-  use_dawn = is_mac
+  use_dawn = false
 }