Fix build with gcc after #313406

Multiple friend declaration is causing warning with gcc 4.8.2 (at least).

Review URL: https://codereview.chromium.org/874933003

Cr-Commit-Position: refs/heads/master@{#313657}

chrome/browser/history/in_memory_history_backend.h

@@ -91,9 +91,6 @@ class InMemoryHistoryBackend : public HistoryServiceObserver {
   void OnKeywordSearchTermDeleted(HistoryService* history_service,
                                   URLID url_id) override;
 
- private:
-  FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, DeleteAll);
-
   // Handler for HISTORY_URL_VISITED and HISTORY_URLS_MODIFIED.
   void OnURLVisitedOrModified(const URLRow& url_row);