Add flag to build old ChromeOS (pre-Aura).

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index d815a4c0703ed59c1dbf4e63626afd4a8cb86122..84c38d96164f9ebebfa191a450220b3112b2e025 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -24,6 +24,9 @@
           # Whether we're building a ChromeOS build.
           'chromeos%': 0,
 
+          # Whether we're building pre-Aura ChromeOS (using GTK).
+          'chromeos_gtk%': 0,
+
           # Whether we are using Views Toolkit
           'toolkit_views%': 0,
 
@@ -45,6 +48,7 @@
         },
         # Copy conditionally-set variables out one scope.
         'chromeos%': '<(chromeos)',
+        'chromeos_gtk%': '<(chromeos_gtk)',
         'use_aura%': '<(use_aura)',
         'use_ash%': '<(use_ash)',
         'use_openssl%': '<(use_openssl)',
@@ -64,21 +68,27 @@
           }],
 
           # Ash and ChromeOS require Aura.
-          ['use_ash==1 or chromeos==1', {
+          ['(use_ash==1 or chromeos==1) and chromeos_gtk==0', {
             'use_aura%': 1,
           }],
 
           # Set default value of toolkit_views based on OS.
-          ['OS=="win" or chromeos==1 or use_aura==1', {
+          ['OS=="win" or chromeos==1 or chromeos_gtk==1 or use_aura==1', {
             'toolkit_views%': 1,
           }, {
             'toolkit_views%': 0,
           }],
+
+          # ChromeOS GTK implies ChromeOS.
+          ['chromeos_gtk==1', {
+            'chromeos%': 1,
+          }],
         ],
       },
 
       # Copy conditionally-set variables out one scope.
       'chromeos%': '<(chromeos)',
+      'chromeos_gtk%': '<(chromeos_gtk)',
       'host_arch%': '<(host_arch)',
       'toolkit_views%': '<(toolkit_views)',
       'use_aura%': '<(use_aura)',
@@ -411,6 +421,7 @@
     'enable_flapper_hacks%': '<(enable_flapper_hacks)',
     'enable_pepper_threading%': '<(enable_pepper_threading)',
     'chromeos%': '<(chromeos)',
+    'chromeos_gtk%': '<(chromeos_gtk)',
     'use_virtual_keyboard%': '<(use_virtual_keyboard)',
     'use_skia_on_mac%': '<(use_skia_on_mac)',
     'use_xi2_mt%':'<(use_xi2_mt)',