Add UMA metric to track usage of sending a mousedown to select elements.

third_party/WebKit/LayoutTests/imported/wpt/web-animations/testcommon.js

Index: third_party/WebKit/LayoutTests/imported/wpt/web-animations/testcommon.js
diff --git a/third_party/WebKit/LayoutTests/imported/wpt/web-animations/testcommon.js b/third_party/WebKit/LayoutTests/imported/wpt/web-animations/testcommon.js
index 5ba03082a9cd65bd7ae916ad99d877e879896484..e375a0f8741788984f9c7be658a3b88c7396c0ff 100644
--- a/third_party/WebKit/LayoutTests/imported/wpt/web-animations/testcommon.js
+++ b/third_party/WebKit/LayoutTests/imported/wpt/web-animations/testcommon.js
@@ -28,15 +28,22 @@ if (!window.assert_times_equal) {
 // creates div element, appends it to the document body and
 // removes the created element during test cleanup
 function createDiv(test, doc) {
+  return createElement(test, 'div', doc);
+}
+
+// creates element of given tagName, appends it to the document body and
+// removes the created element during test cleanup
+// if tagName is null or undefined, returns div element
+function createElement(test, tagName, doc) {
   if (!doc) {
     doc = document;
   }
-  var div = doc.createElement('div');
-  doc.body.appendChild(div);
+  var element = doc.createElement(tagName || 'div');
+  doc.body.appendChild(element);
   test.add_cleanup(function() {
-    div.remove();
+    element.remove();
   });
-  return div;
+  return element;
 }
 
 // Creates a style element with the specified rules, appends it to the document