[Chromecast] Replace architecure-based conditionals with flag.

build/config/chromecast_build.gni

Index: build/config/chromecast_build.gni
diff --git a/build/config/chromecast_build.gni b/build/config/chromecast_build.gni
index b3d1bc23bafc99561f27f5239a2782a845757421..5e3b7af0150af5c483d42ddf53b487fc2095e5f3 100644
--- a/build/config/chromecast_build.gni
+++ b/build/config/chromecast_build.gni
@@ -9,6 +9,14 @@ declare_args() {
   # Linux and Android.
   is_chromecast = false
 
+  # True if Chromecast build is targeted for linux desktop. This type of build
+  # is useful for testing and development, but currently supports only a subset
+  # of Cast functionality. Though this defaults to true for x86 Linux devices,
+  # this should be overriden manually for an embedded x86 build.
+  # TODO(slan): Remove instances of this when x86 is a fully supported platform.
+  is_cast_desktop_build =
+      is_chromecast && is_linux && (target_cpu == "x86" || target_cpu == "x64")
+
   # Set this true for an audio-only Chromecast build.
   disable_display = false
 }
@@ -17,5 +25,6 @@ declare_args() {
 assert(is_linux || is_android || !is_chromecast,
        "Chromecast builds are not supported on $target_os")
 
-# Assert that disable_display is not true on a non-Chromecast build.
-assert(!disable_display || is_chromecast)
+# Assert that disable_display and is_cast_desktop_build are both false on a
+# non-Chromecast build.
+assert(is_chromecast || !(disable_display || is_cast_desktop_build))