Move enable extensions define to a build flag.

chrome/common/features.gni

Index: chrome/common/features.gni
diff --git a/chrome/common/features.gni b/chrome/common/features.gni
index 6f90abe33fc95081a3d41c6b506383504b3c4487..2c9a9beb8b1071683405f2a1a152bfcb6381ee3b 100644
--- a/chrome/common/features.gni
+++ b/chrome/common/features.gni
@@ -6,6 +6,7 @@ import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/features.gni")
+import("//extensions/features/features.gni")
 import("//net/features.gni")
 import("//printing/features/features.gni")
 
@@ -80,6 +81,7 @@ _enable_printing = enable_basic_printing || enable_print_preview
 chrome_grit_defines = [
   "enable_app_list=$enable_app_list",
   "enable_background=$enable_background",
+  "enable_extensions=$enable_extensions",
   "enable_google_now=$enable_google_now",
   "enable_hangout_services_extension=$enable_hangout_services_extension",
   "enable_hotwording=$enable_hotwording",