[Chromecast] Replace architecure-based conditionals with flag.

chromecast/chromecast_tests.gypi

Index: chromecast/chromecast_tests.gypi
diff --git a/chromecast/chromecast_tests.gypi b/chromecast/chromecast_tests.gypi
index c889bbc86e7bec6b8c73e14f8f7d330377a48173..7457260c2f05f2eddd326de12e91efe389ecb460 100644
--- a/chromecast/chromecast_tests.gypi
+++ b/chromecast/chromecast_tests.gypi
@@ -111,7 +111,7 @@
         '../url/url.gyp:url_unittests',
       ],
       'conditions': [
-        ['target_arch=="arm" and OS!="android"', {
+        ['is_cast_desktop_build==0 and OS!="android"', {
           'variables': {
             'filters': [
               # Run net_unittests first to avoid random failures due to slow python startup