Add initial version of retainers heap profile.

src/log.h

Index: src/log.h
diff --git a/src/log.h b/src/log.h
index 89f6cdb3915d0e69b10e4a16e61a1ce783fd6893..17f7e00651b05ae498a7020b6164a8bd4cb102c0 100644
--- a/src/log.h
+++ b/src/log.h
@@ -221,6 +221,7 @@ class Logger {
   static void HeapSampleItemEvent(const char* type, int number, int bytes);
   static void HeapSampleJSConstructorEvent(const char* constructor,
                                            int number, int bytes);
+  static void HeapSampleJSRetainersEvent(const char* event);
   static void HeapSampleStats(const char* space, const char* kind,
                               int capacity, int used);