[Extensions] Handle null ScriptContexts in GetModuleSystem()

extensions/renderer/script_context_set.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_RENDERER_SCRIPT_CONTEXT_SET_H_
 #define EXTENSIONS_RENDERER_SCRIPT_CONTEXT_SET_H_
 
 #include <stddef.h>
 
 #include <set>
@@ skipping 57 matching lines @@
 
   // Gets the ScriptContext corresponding to the specified
   // v8::Context or NULL if no such context exists.
   ScriptContext* GetByV8Context(const v8::Local<v8::Context>& context) const;
   // Static equivalent of the above.
   static ScriptContext* GetContextByV8Context(
       const v8::Local<v8::Context>& context);
 
   // Returns the ScriptContext corresponding to the V8 context that created the
   // given |object|.
+  // Note: The provided |object| may belong to a v8::Context in another frame,
+  // as can happen when a parent frame uses an object of an embedded iframe.
+  // In this case, there may be no associated ScriptContext, since the child
+  // frame can be hosted in another process. Thus, callers of this need to
+  // null-check the result (and should also always check whether or not the
+  // context has access to the other context).
   static ScriptContext* GetContextByObject(const v8::Local<v8::Object>& object);
 
   // Synchronously runs |callback| with each ScriptContext that belongs to
   // |extension_id| in |render_frame|.
   //
   // An empty |extension_id| will match all extensions, and a null
   // |render_frame| will match all render views, but try to use the inline
   // variants of these methods instead.
   void ForEach(const std::string& extension_id,
                content::RenderFrame* render_frame,
@@ skipping 46 matching lines @@
 
   // The set of all ScriptContexts we own.
   std::set<ScriptContext*> contexts_;
 
   DISALLOW_COPY_AND_ASSIGN(ScriptContextSet);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_SCRIPT_CONTEXT_SET_H_