[chromecast] Fix broken build when enable_chromecast_webui is false.

Caused by https://codereview.chromium.org/2318803002/

BUG=internal b/30408323

Review-Url: https://codereview.chromium.org/2316923002
Cr-Commit-Position: refs/heads/master@{#416867}

diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index 9e5a6ba..6f71705 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni

@@ -22,6 +22,11 @@
   # it will evaluate to 0. Overriding this when is_debug=false is useful for
   # doing engineering builds.
   cast_is_debug = is_debug
+
+  # If true, Chromecast WebUI resources are included in a build.
+  # TODO(antz): default to false for audio-only builds, might need further
+  # clean up (b/27119303)
+  enable_chromecast_webui = !is_cast_audio_only && !is_android
 }
 
 declare_args() {