Add DCHECK to ensure IndexedDBDispatcher doesn't get re-created.

content/common/indexed_db/indexed_db_dispatcher.h

Index: content/common/indexed_db/indexed_db_dispatcher.h
diff --git a/content/common/indexed_db/indexed_db_dispatcher.h b/content/common/indexed_db/indexed_db_dispatcher.h
index f48e7c2da3a3980420935ae4ba087c33cfa5778d..761c0357d950f2cb0b195a7e9a9b154891ca7c94 100644
--- a/content/common/indexed_db/indexed_db_dispatcher.h
+++ b/content/common/indexed_db/indexed_db_dispatcher.h
@@ -49,6 +49,11 @@ CONTENT_EXPORT extern const size_t kMaxIDBValueSizeInBytes;
 class CONTENT_EXPORT IndexedDBDispatcher
     : public webkit_glue::WorkerTaskRunner::Observer {
  public:
+  // Constructor made public to allow RenderThreadImpl to own a copy without
+  // failing a NOTREACHED in ThreadSpecificInstance in tests that instantiate
+  // two copies of RenderThreadImpl on the same thread.  Everyone else probably
+  // wants to use ThreadSpecificInstance().
+  IndexedDBDispatcher();
   virtual ~IndexedDBDispatcher();
   static IndexedDBDispatcher* ThreadSpecificInstance();
 
@@ -209,7 +214,6 @@ class CONTENT_EXPORT IndexedDBDispatcher
  private:
   FRIEND_TEST_ALL_PREFIXES(IndexedDBDispatcherTest, ValueSizeTest);
 
-  IndexedDBDispatcher();
   // IDBCallback message handlers.
   void OnSuccessNull(int32 response_id);
   void OnSuccessIDBDatabase(int32 thread_id,