WebUI: Fix crash while loading scripts for webview.[add/remove]ContentScripts API.

extensions/browser/user_script_loader.h

 // 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.
 
 #ifndef EXTENSIONS_BROWSER_USER_SCRIPT_LOADER_H_
 #define EXTENSIONS_BROWSER_USER_SCRIPT_LOADER_H_
 
 #include <map>
 #include <memory>
 #include <set>
@@ skipping 77 matching lines @@
 
   // Pickle user scripts and return pointer to the shared memory.
   static std::unique_ptr<base::SharedMemory> Serialize(
       const extensions::UserScriptList& scripts);
 
   // Adds or removes observers.
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
  protected:
-  // Allows the derived classes have different ways to load user scripts.
+  // Allows the derived classes to have different ways to load user scripts.
+  // This may not be synchronous with the calls to Add/Remove/Clear scripts.
   virtual void LoadScripts(std::unique_ptr<UserScriptList> user_scripts,
                            const std::set<HostID>& changed_hosts,
                            const std::set<int>& added_script_ids,
                            LoadScriptsCallback callback) = 0;
 
   // Sets the flag if the initial set of hosts has finished loading; if it's
   // set to be true, calls AttempLoad() to bootstrap.
   void SetReady(bool ready);
 
   content::BrowserContext* browser_context() const { return browser_context_; }
@@ skipping 72 matching lines @@
   base::ObserverList<Observer> observers_;
 
   base::WeakPtrFactory<UserScriptLoader> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(UserScriptLoader);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_USER_SCRIPT_LOADER_H_