Add a comment why we use HeapVector for a map in EventListenerMap.

third_party/WebKit/Source/core/events/EventListenerMap.h

Index: third_party/WebKit/Source/core/events/EventListenerMap.h
diff --git a/third_party/WebKit/Source/core/events/EventListenerMap.h b/third_party/WebKit/Source/core/events/EventListenerMap.h
index 37851fc0c22a5493dbc1f3c207ba989fa73c8510..77c61a5f05b8d8cf287663077edb914e31fda9cf 100644
--- a/third_party/WebKit/Source/core/events/EventListenerMap.h
+++ b/third_party/WebKit/Source/core/events/EventListenerMap.h
@@ -71,6 +71,10 @@ private:
 
     void assertNoActiveIterators();
 
+    // We use HeapVector instead of HeapHashMap because
+    //  - HeapVector is much more space efficient than HeapHashMap.
+    //  - An EventTarget rarely has event listeners for many event types, and
+    //    HeapVector is faster in such cases.
     HeapVector<std::pair<AtomicString, Member<EventListenerVector>>, 2> m_entries;
 
 #if ENABLE(ASSERT)