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

third_party/WebKit/LayoutTests/imported/wpt/pointerevents/pointerevent_styles.css

+#innerFrame {
+position: absolute;
+top: 250px;
+left: 200px;
+height: 100px;
+width: 100px;
+}
+
 .spacer {
 height: 100px;
 }
 
 #square1 {
+top: 250px;
+left: 100px;
 background: black;
-top: 150px;
-left: 100px;
+}
+
+#square2 {
+top: 50px;
+left: 30px;
+visibility: hidden;
+background: red;
 }
 
 .square {
 height: 20px;
 width: 20px;
 position: absolute;
 padding: 0px;
 }
 
 #target0 {
 background: black;
 color: white;
 white-space: nowrap;
 overflow-y: auto;
 overflow-x: auto;
 }
 
 #target1 {
 background: purple;
 color: white;
 white-space: nowrap;
 overflow-y: auto;
 overflow-x: auto;
 }
 
+#scrollTarget {
+  background: darkblue;
+}
+
 .touchActionNone {
 touch-action: none;
 }
 
 #innerframe {
 width: 90%;
 margin: 10px;
 margin-left: 10%;
 height: 200px;
 }
@@ skipping 40 matching lines @@
 #listener {
 background: orange;
 border: 1px solid orange;
 position: absolute;
 top: -100px;
 }
 
 body.scrollable {
 min-height: 5000px;
 }