Add UMA measuring pan scrolling usage.

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

Index: third_party/WebKit/LayoutTests/fast/events/panScroll-use-count.html
diff --git a/third_party/WebKit/LayoutTests/fast/events/panScroll-use-count.html b/third_party/WebKit/LayoutTests/fast/events/panScroll-use-count.html
new file mode 100644
index 0000000000000000000000000000000000000000..c01c78f3b06e4a1f898a3ded8513985eb9a51aac
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/fast/events/panScroll-use-count.html
@@ -0,0 +1,35 @@
+<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 aroudn mouse pointer for scrolling, then release middle button to stop scrolling.

[tdresser, 2016/09/07 13:41:02]

around

+<div id="scrollable"></div>
+</div>
+<div id="console"></div>
+</body>
+<script>
+test(function() {
+	if (!window.eventSender)

[tdresser, 2016/09/07 13:41:02]

Indentation is wonky.

+		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>