extensions: Check |enable_extensions| flag before depending on extension code.

apps/apps.gypi

Index: apps/apps.gypi
diff --git a/apps/apps.gypi b/apps/apps.gypi
index 7badfe3429e5e4ebfae7d291241ca8672fc4c59b..130cb8cda205bea5a44edbf606f810ce95f2e01e 100644
--- a/apps/apps.gypi
+++ b/apps/apps.gypi
@@ -75,7 +75,6 @@
         ],
         ['toolkit_views==1', {
           'dependencies': [
-            '../extensions/extensions.gyp:extensions_browser',
             '../ui/strings/ui_strings.gyp:ui_strings',
             '../ui/views/views.gyp:views',
           ],
@@ -84,6 +83,11 @@
             ['exclude', 'ui/views/'],
           ],
         }],
+        ['toolkit_views==1 and enable_extensions==1', {
+          'dependencies': [
+            '../extensions/extensions.gyp:extensions_browser',
+          ],
+        }],
       ],
       # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
       'msvs_disabled_warnings': [ 4267, ],