[tracing] Fix MemoryDumpManager testing instance teardown pattern.

base/trace_event/memory_dump_manager.h

Index: base/trace_event/memory_dump_manager.h
diff --git a/base/trace_event/memory_dump_manager.h b/base/trace_event/memory_dump_manager.h
index fbc71d5e006e0048a2f09c2d83413f2e1676fba9..1a22e61b34a5640ee5291458b29da6b14e4ceaef 100644
--- a/base/trace_event/memory_dump_manager.h
+++ b/base/trace_event/memory_dump_manager.h
@@ -50,17 +50,17 @@ class BASE_EXPORT MemoryDumpManager : public TraceLog::EnabledStateObserver {
   void OnTraceLogDisabled() override;
 
  private:
+  friend struct DefaultDeleter<MemoryDumpManager>;  // For the testing instance.
   friend struct DefaultSingletonTraits<MemoryDumpManager>;
   friend class MemoryDumpManagerTest;
 
   static const char kTraceCategory[];
 
+  static void SetInstanceForTesting(MemoryDumpManager* instance);
+
   MemoryDumpManager();
   virtual ~MemoryDumpManager();
 
-  // Tears down the singleton instance.
-  static void DeleteForTesting();
-
   // Broadcasts the dump requests to the other processes.
   void BroadcastDumpRequest();