[Closure Externs] Generate closure compiler externs as part of compile

chrome/common/extensions/api/BUILD.gn

Index: chrome/common/extensions/api/BUILD.gn
diff --git a/chrome/common/extensions/api/BUILD.gn b/chrome/common/extensions/api/BUILD.gn
index b419b51de87ddd3c522a2fabf326119ceb24969c..73e3cbbf28a42411afa230e00589d60dfef9d334 100644
--- a/chrome/common/extensions/api/BUILD.gn
+++ b/chrome/common/extensions/api/BUILD.gn
@@ -7,6 +7,7 @@ import("//build/config/ui.gni")
 import("//chrome/common/features.gni")
 import("//extensions/features/features.gni")
 import("//media/media_options.gni")
+import("//tools/json_schema_compiler/externs.gni")
 import("//tools/json_schema_compiler/json_features.gni")
 import("//tools/json_schema_compiler/json_schema_api.gni")
 
@@ -274,3 +275,7 @@ group("extensions_features") {
     ":permission_features",
   ]
 }
+
+closure_externs("api_externs") {
+  sources = [ "developer_private.idl" ]
+}