Move enable extensions define to a build flag.

chrome/browser/BUILD.gn

Index: chrome/browser/BUILD.gn
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 2d45cd4027358193598d8a55c9151e3056a7e517..c0e190207300afc7ff2634f81b639067bcf9fd89 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -9,6 +9,7 @@ import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
 import("//components/os_crypt/features.gni")
+import("//extensions/features/features.gni")
 import("//media/media_options.gni")
 import("//net/features.gni")
 import("//ppapi/features/features.gni")
@@ -1432,6 +1433,7 @@ split_static_library("browser") {
     "//device/power_save_blocker",
     "//device/usb/mojo",
     "//device/usb/public/interfaces",
+    "//extensions/features",
     "//google_apis",
     "//gpu/config",
     "//media",