[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..b2259704983060e0b24006efedeceb12b60e947f 100644
--- a/build/config/chromecast_build.gni
+++ b/build/config/chromecast_build.gni
@@ -2,8 +2,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# This block should only contain Chromecast-specific build arguments used by
-# by components outside of //chromecast.
+# The args declared in this file should be referenced by components outside of
+# //chromecast. Args needed only in //chromecast should be declared in
+# //chromecast/chromecast.gni.
 declare_args() {
   # Set this true for a Chromecast build. Chromecast builds are supported on
   # Linux and Android.
@@ -13,9 +14,23 @@ declare_args() {
   disable_display = false
 }
 
+# Note(slan): This arg depends on the value of is_chromecast, and thus must be
+# declared in a separate block. These blocks can be combined when/if
+# crbug.com/542846 is resolved.
+declare_args() {
+  # 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 && target_os == "linux" &&
+                          (target_cpu == "x86" || target_cpu == "x64")
+}
+
 # Assert that Chromecast is being built for a supported platform.
 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))