Move enable extensions define to a build flag.

extensions/browser/BUILD.gn

Index: extensions/browser/BUILD.gn
diff --git a/extensions/browser/BUILD.gn b/extensions/browser/BUILD.gn
index 0e56510cb188a5602060faabbb0f6ae4411d49e6..1fe3f5eecb8109e0aa2115734715812c340e9bb4 100644
--- a/extensions/browser/BUILD.gn
+++ b/extensions/browser/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
+import("//extensions/features/features.gni")
 
 source_set("browser") {
   sources = []
@@ -25,6 +26,7 @@ source_set("browser") {
     "//extensions/browser/api:api_registration",
     "//extensions/common",
     "//extensions/common/api",
+    "//extensions/features",
     "//extensions/strings",
     "//google_apis",
     "//ui/display",
@@ -436,6 +438,7 @@ source_set("unit_tests") {
     "//extensions:test_support",
     "//extensions/common",
     "//extensions/common/api",
+    "//extensions/features",
     "//ipc:test_support",
     "//net:test_support",
     "//third_party/leveldatabase",