Fix missing GN dependencies.

extensions/BUILD.gn

 # Copyright 2014 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.
 
 import("//extensions/extensions.gni")
 import("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
 assert(enable_extensions)
@@ skipping 102 matching lines @@
     "$root_gen_dir/ui/resources/ui_resources_100_percent.pak",
     "$root_gen_dir/ui/strings/app_locale_settings_en-US.pak",
     "$root_gen_dir/ui/strings/ui_strings_en-US.pak",
   ]
 
   output = "$root_out_dir/extensions_shell_and_test.pak"
 
   deps = [
     ":extensions_resources",
     "//content:resources",
+    "//content/app/strings",
     "//content/browser/devtools:devtools_resources",
     "//content/shell:resources",
     "//extensions/shell:resources",
     "//extensions/strings",
     "//third_party/WebKit/public:image_resources",
     "//third_party/WebKit/public:resources",
     "//ui/resources",
     "//ui/strings",
   ]
 }
@@ skipping 82 matching lines @@
   ]
 
   if (is_win) {
     deps += [ "//base/allocator" ]
   }
 
   if (is_mac) {
     deps += [ "shell:app_shell" ]  # Needed for App Shell.app's Helper.
   }
 }