Rename is_chromeos in chrome/src.

build/config/ui.gni

Index: build/config/ui.gni
diff --git a/build/config/ui.gni b/build/config/ui.gni
index ca2f26521fc504148d344d6845845b3cfec42999..ca3c554ce5d0e82cc19b4988794c854e113aba11 100644
--- a/build/config/ui.gni
+++ b/build/config/ui.gni
@@ -14,6 +14,8 @@
 # required. See the declare_args block of BUILDCONFIG.gn for advice on how
 # to set up feature flags.
 
+import("//build/config/features.gni")
+
 declare_args() {
   # Indicates if Ash is enabled. Ash is the Aura Shell which provides a
   # desktop-like environment for Aura. Requires use_aura = true
@@ -31,7 +33,7 @@ declare_args() {
   use_aura = is_win || is_linux
 
   # True means the UI is built using the "views" framework.
-  toolkit_views = is_mac || is_win || is_chromeos || use_aura
+  toolkit_views = is_mac || is_win || use_cros_fe || use_aura
 
   # Whether the entire browser uses toolkit-views on Mac instead of Cocoa.
   mac_views_browser = false
@@ -70,4 +72,12 @@ 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
+enable_topchrome_md = use_cros_fe || is_win || is_linux
+
+if (is_chromeos_os) {
+  assert(use_cros_fe, "ChromeOS targetting requires ChromeOS UI")
+}
+
+assert(use_cros_fe == is_chromeos,
+       "is_chromeos and use_cros_fe are expected to be aliases, " +
+           "old and new, respectively")