Move enable extensions define to a build flag.

extensions/common/api/BUILD.gn

Index: extensions/common/api/BUILD.gn
diff --git a/extensions/common/api/BUILD.gn b/extensions/common/api/BUILD.gn
index ee6d65df7519caddf92eb88bbb539042f1175ca2..108e9a72abdbf91b8c61af3262aef3bf8277678a 100644
--- a/extensions/common/api/BUILD.gn
+++ b/extensions/common/api/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/config/features.gni")
 import("//build/json_schema_api.gni")
 import("//extensions/common/api/schema.gni")
+import("//extensions/features/features.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 
 assert(enable_extensions)
@@ -18,6 +19,10 @@ json_schema_api("generated_api") {
   uncompiled_sources = extensions_api_uncompiled_sources
   uncompiled_bundle_schema_sources =
       extensions_api_uncompiled_bundle_schema_sources
+  deps = [
+    "//base",
+    "//extensions/features",
+  ]
 }
 
 mojom("mojom") {
@@ -32,5 +37,6 @@ group("api") {
   public_deps = [
     ":generated_api",
     ":mojom",
+    "//extensions/features",
   ]
 }