Add aggregated memory histograms.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 71ee26e2cecc76f9784c8790947db2a4c226733f..aaf7874de0c985451eb38aae8d1405377f29df4f 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1146,6 +1146,8 @@ class Heap {
   bool IdleNotification(double deadline_in_seconds);
   bool IdleNotification(int idle_time_in_ms);
 
+  double MonotonicallyIncreasingTimeInMs();
+
   // Declare all the root indices.  This defines the root list order.
   enum RootListIndex {
 #define ROOT_INDEX_DECLARATION(type, name, camel_name) k##camel_name##RootIndex,