Avoid crashing if MHTMLGenerationManager::JobFinished is called twice.

content/browser/download/mhtml_generation_manager.h

 // Copyright (c) 2012 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 CONTENT_BROWSER_DOWNLOAD_MHTML_GENERATION_MANAGER_H_
 #define CONTENT_BROWSER_DOWNLOAD_MHTML_GENERATION_MANAGER_H_
 
 #include <stdint.h>
 
 #include <map>
 #include <set>
@@ skipping 10 matching lines @@
 class FilePath;
 }
 
 namespace content {
 
 class RenderFrameHostImpl;
 class WebContents;
 
 // The class and all of its members live on the UI thread.  Only static methods
 // are executed on other threads.
+//
+// MHTMLGenerationManager is a singleton.  Each call to SaveMHTML method creates
+// a new instance of MHTMLGenerationManager::Job that tracks generation of a
+// single MHTML file.
 class MHTMLGenerationManager {
  public:
   static MHTMLGenerationManager* GetInstance();
 
   // GenerateMHTMLCallback is called to report completion and status of MHTML
   // generation.  On success |file_size| indicates the size of the
   // generated file.  On failure |file_size| is -1.
   typedef base::Callback<void(int64_t file_size)> GenerateMHTMLCallback;
 
   // Instructs the RenderFrames in |web_contents| to generate a MHTML
@@ skipping 46 matching lines @@
   IDToJobMap id_to_job_;
 
   int next_job_id_;
 
   DISALLOW_COPY_AND_ASSIGN(MHTMLGenerationManager);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_MHTML_GENERATION_MANAGER_H_