Avoid crashing if MHTMLGenerationManager::JobFinished is called twice.

content/browser/download/mhtml_generation_manager.h

Index: content/browser/download/mhtml_generation_manager.h
diff --git a/content/browser/download/mhtml_generation_manager.h b/content/browser/download/mhtml_generation_manager.h
index 39dad54b0f43083dec24a5990166de6591d13659..50d5bebc702bc782b3535cd0b19508558f67f240 100644
--- a/content/browser/download/mhtml_generation_manager.h
+++ b/content/browser/download/mhtml_generation_manager.h
@@ -28,6 +28,10 @@ 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();