[UserScript] Do not create SubstitutionMap if not necessary.

extensions/browser/extension_user_script_loader.cc

 // Copyright 2015 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.
 
 #include "extensions/browser/extension_user_script_loader.h"
 
 #include <stddef.h>
 
 #include <set>
 #include <string>
@@ skipping 113 matching lines @@
       iter->second.first, host_id.id(), iter->second.second);
 }
 
 void LoadUserScripts(UserScriptList* user_scripts,
                      const ExtensionUserScriptLoader::HostsInfo& hosts_info,
                      const std::set<int>& added_script_ids,
                      const scoped_refptr<ContentVerifier>& verifier) {
   for (UserScript& script : *user_scripts) {
     if (added_script_ids.count(script.id()) == 0)
       continue;
-    std::unique_ptr<SubstitutionMap> localization_messages(
-        GetLocalizationMessages(hosts_info, script.host_id()));
     for (UserScript::File& script_file : script.js_scripts()) {
       if (script_file.GetContent().empty())
         LoadScriptContent(script.host_id(), &script_file, nullptr, verifier);
     }
-    for (UserScript::File& script_file : script.css_scripts()) {
-      if (script_file.GetContent().empty())
-        LoadScriptContent(script.host_id(), &script_file,
-                          localization_messages.get(), verifier);
+    if (script.css_scripts().size() > 0) {
+      std::unique_ptr<SubstitutionMap> localization_messages(
+          GetLocalizationMessages(hosts_info, script.host_id()));
+      for (UserScript::File& script_file : script.css_scripts()) {
+        if (script_file.GetContent().empty()) {
+          LoadScriptContent(script.host_id(), &script_file,
+                            localization_messages.get(), verifier);
+        }
+      }
     }
   }
 }
 
 void LoadScriptsOnFileThread(
     std::unique_ptr<UserScriptList> user_scripts,
     const ExtensionUserScriptLoader::HostsInfo& hosts_info,
     const std::set<int>& added_script_ids,
     const scoped_refptr<ContentVerifier>& verifier,
     UserScriptLoader::LoadScriptsCallback callback) {
@@ skipping 78 matching lines @@
     const Extension* extension,
     UnloadedExtensionInfo::Reason reason) {
   hosts_info_.erase(HostID(HostID::EXTENSIONS, extension->id()));
 }
 
 void ExtensionUserScriptLoader::OnExtensionSystemReady() {
   SetReady(true);
 }
 
 }  // namespace extensions