Decompose //extensions/browser/BUILD.gn

extensions/browser/mojo/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("cpp") {
+source_set("mojo") {
   sources = [
-    "resource_loader.cc",
-    "resource_loader.h",
+    "keep_alive_impl.cc",
+    "keep_alive_impl.h",
+    "service_registration.cc",
+    "service_registration.h",
+    "stash_backend.cc",
+    "stash_backend.h",
   ]
 
   deps = [
-    "//base",
-    "//components/filesystem/public/interfaces",
-    "//mojo/public/cpp/bindings",
-    "//mojo/public/cpp/system",
+    "//extensions/common:mojo",
     "//services/shell/public/cpp",
   ]
 }