Rename is_chromeos.

build/config/ui.gni

Index: build/config/ui.gni
diff --git a/build/config/ui.gni b/build/config/ui.gni
index ca2f26521fc504148d344d6845845b3cfec42999..1ffc21f8b0acf539e175b4b7c522c9af55b8c892 100644
--- a/build/config/ui.gni
+++ b/build/config/ui.gni
@@ -15,6 +15,11 @@
 # to set up feature flags.
 
 declare_args() {
+  # This indicates whether the chromeos interfaces and features should be
+  # made available.  Contrast with whether we are targetting the embedded
+  # OS.  That flag controls the linking, flags, filesystems, etc. we expect.
+  use_cros_ui = is_chromeos
+
   # Indicates if Ash is enabled. Ash is the Aura Shell which provides a
   # desktop-like environment for Aura. Requires use_aura = true
   use_ash = is_win || is_linux
@@ -71,3 +76,11 @@ use_clipboard_aurax11 = is_linux && use_aura && use_x11
 enable_hidpi = is_mac || is_win || is_linux
 
 enable_topchrome_md = is_chromeos || is_win || is_linux
+
+if (is_chromeos_os) {
+  assert(use_cros_ui, "ChromeOS targetting requires ChromeOS UI")
+}
+
+assert(use_cros_ui == is_chromeos,
+       "is_chromeos and use_cros_ui are expected to be aliases, " +
+           "old and new, respectively")