Move enable extensions define to a build flag.

chrome/common/BUILD.gn

Index: chrome/common/BUILD.gn
diff --git a/chrome/common/BUILD.gn b/chrome/common/BUILD.gn
index 96ac6a7ddf553ac713b23c2485684788a9be90c5..443a67cee597410b5ebd304c4d59e1927b925605 100644
--- a/chrome/common/BUILD.gn
+++ b/chrome/common/BUILD.gn
@@ -6,6 +6,7 @@ import("//build/buildflag_header.gni")
 import("//build/util/process_version.gni")
 import("//chrome/common/features.gni")
 import("//chrome/process_version_rc_template.gni")  # For branding_file_path.
+import("//extensions/features/features.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//tools/grit/grit_rule.gni")
 
@@ -209,6 +210,7 @@ static_library("common") {
     "//content/public/common",
     "//crypto",
     "//extensions/common:common_constants",
+    "//extensions/features",
     "//gin",
     "//google_apis",
     "//gpu/command_buffer/service",
@@ -568,6 +570,7 @@ static_library("constants") {
   public_deps = [
     ":features",
     "//content/public/common:result_codes",
+    "//extensions/features",
     "//printing/features",
   ]
   deps = [