New build flag system, convert Google Now flag

chrome/common/features.gni

Index: chrome/common/features.gni
diff --git a/chrome/common/features.gni b/chrome/common/features.gni
new file mode 100644
index 0000000000000000000000000000000000000000..2e92745ca3b5ae136b5ec6bc9eaa1ee73abd98a1
--- /dev/null
+++ b/chrome/common/features.gni
@@ -0,0 +1,12 @@
+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# This is the GN version of //chrome/chrome_features.gypi.
+# Please keep in sync!
+
+declare_args() {
+  enable_google_now = !is_ios && !is_android
+}
+
+chrome_grit_defines = [ "enable_google_now=$enable_google_now" ]