Implement a Hit Test Cache.

LayoutTests/fast/events/hit-test-counts.html

Index: LayoutTests/fast/events/hit-test-counts.html
diff --git a/LayoutTests/fast/events/hit-test-counts.html b/LayoutTests/fast/events/hit-test-counts.html
index cfef9cf58136aab4bdc53b2c9fd47f0be9a6635e..4e39d6d9fba12e0feb2929439384ecdbec356cee 100644
--- a/LayoutTests/fast/events/hit-test-counts.html
+++ b/LayoutTests/fast/events/hit-test-counts.html
@@ -43,6 +43,16 @@ function hitTestCountDelta(doc)
     return newCount - lastCount;
 }
 
+function hitTestCacheHitsDelta(doc)
+{
+    var lastCount = 0;
+    if ('lastHitTestCacheHits' in doc)
+      lastCount = doc.lastHitTestCacheHits;
+    var newCount = internals.hitTestCacheHits(doc);
+    doc.lastHitTestCacheHits = newCount;
+    return newCount - lastCount;
+}
+
 function logCounts(label, documents, multiTapNotification, eventSenderFunction)
 {
     if (eventSenderFunction)
@@ -50,7 +60,7 @@ function logCounts(label, documents, multiTapNotification, eventSenderFunction)
 
     var countStr = '';
     for(var i = 0; i < documents.length; i++)
-        countStr += ' ' + hitTestCountDelta(documents[i]);
+        countStr += ' ' + hitTestCountDelta(documents[i]) + "/" + hitTestCacheHitsDelta(documents[i]);

[Rick Byers, 2015/06/05 20:48:59]

nit: I'd find this more readable if it was "<misses>+<hits>" instead of
"total/hits".  It's really the misses number I care most about in this test -
would be nice to be able to scan without doing the math for each result.

[dtapuska, 2015/06/09 18:21:23]

Done.

 
     if (multiTapNotification) {
       debug(label + ':' + eventCounts[label]
@@ -64,8 +74,10 @@ function logCounts(label, documents, multiTapNotification, eventSenderFunction)
 
 function clearCounts(documents)
 {
-    for(var i = 0; i < documents.length; i++)
+    for(var i = 0; i < documents.length; i++) {
         documents[i].lastHitTestCount = internals.hitTestCount(documents[i]);
+        documents[i].lastHitTestCacheHits = internals.hitTestCacheHits(documents[i]);
+    }
 }
 
 function sendEvents(targetX, targetY, documents, multiTapNotification)