Cherry-pick ad21476. rdar://problem/105010273

    EventListenerMap's m_entries vector wastes a lot of vector capacity
    https://bugs.webkit.org/show_bug.cgi?id=251697
    <rdar://problem/105010273>

    Reviewed by Mark Lam.

    EventListenerMap's vector wastes capacity, since it allocates 16 slots but uses on average 1.63 of them; most contain only 1.
    So set the initial capacity to 4. This saves about 54K on nytimes.com. It measures as perf neutral on Speedometer.

    * Source/WebCore/dom/EventListenerMap.h:

    Canonical link: https://commits.webkit.org/259920@main

Canonical link: https://commits.webkit.org/259548.226@safari-7615-branch

Source/WebCore/dom/EventListenerMap.h

@@ -74,7 +74,7 @@ class EventListenerMap {
     Lock& lock() { return m_lock; }
 
 private:
-    Vector<std::pair<AtomString, EventListenerVector>> m_entries;
+    Vector<std::pair<AtomString, EventListenerVector>, 0, CrashOnOverflow, 4> m_entries;
     Lock m_lock;
 };