Add UMA measuring pan scrolling usage.

third_party/WebKit/LayoutTests/fast/events/panScroll-use-count.html

+<style type="text/css">
+#scrollable {
+    height: 1000px;
+    overflow: auto;
+    border: solid 3px #cc0000;
+    font-size: 80px;
+}
+</style>
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<div id="container">
+<p id="description"></p>
+For manual testing, hold middle button in scrollable and move around mouse pointer for scrolling, then release middle button to stop scrolling.
+<div id="scrollable"></div>
+</div>
+<div id="console"></div>
+</body>
+<script>
+test(function() {
+    if (!window.eventSender)
+        return;
+    var middleButton = 1;
+    var panScrollRadius = 15;
+    var PanScrollingStart = 1551;
+    var scrollable = document.getElementById("scrollable");
+    var startX = scrollable.offsetLeft + 5;
+    var startY = scrollable.offsetTop + 5;
+    var endX = startX;
+    var endY = scrollable.offsetTop + panScrollRadius + 6;
+    eventSender.mouseMoveTo(startX, startY);
+    eventSender.mouseDown(middleButton);
+    eventSender.mouseMoveTo(endX, endY);
+    assert_true(internals.isUseCounted(document, PanScrollingStart));
+})
+</script>