Move enable extensions define to a build flag.

chrome/browser/extensions/BUILD.gn

Index: chrome/browser/extensions/BUILD.gn
diff --git a/chrome/browser/extensions/BUILD.gn b/chrome/browser/extensions/BUILD.gn
index 5688605c3118eea2aa551e03a1d9ec8b2ba9baa5..965847ac5e042dcf7bb2479b7ceb3dd3b52d3a5a 100644
--- a/chrome/browser/extensions/BUILD.gn
+++ b/chrome/browser/extensions/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//chrome/common/features.gni")
+import("//extensions/features/features.gni")
 
 assert(enable_extensions)
 
@@ -887,6 +888,7 @@ static_library("extensions") {
     "//extensions/browser",
     "//extensions/browser/api:api_registration",
     "//extensions/common/api",
+    "//extensions/features",
     "//extensions/strings",
     "//net",
     "//printing/features",