[Chromecast] Don't compile Dawn for Chromecast

Since Chromecast doesn't use Dawn, we can trim the binary size by
excluding Dawn from compilation.

Bug: internal b/174509097
Test: CQ
Change-Id: Ibb32066ad3bb59852dfdb04c9ba5484c29283c8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2573637Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833983}

ui/gl/features.gni

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/chromecast_build.gni")
 import("//build/config/chromeos/ui_mode.gni")
 
 declare_args() {
@@ -16,7 +17,7 @@ declare_args() {
 
   # Should Dawn support be compiled to back the WebGPU implementation?
   # Also controls linking Dawn depedencies in such as SPIRV-Tools/SPIRV-Cross.
-  use_dawn = is_mac || is_win || is_chromeos || is_linux
+  use_dawn = is_mac || is_win || is_chromeos || (is_linux && !is_chromecast)
 
   # Should Dawn test binaries (unittests, end2end_tests, perf_tests) be built?
   # Independent of use_dawn, which controls whether Dawn is used in Chromium.