Add UMA for selection drag autoscroll.

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

+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<div id="scrollable" style="height:100px; overflow: auto">
+    <p>This line is longer to ensure selection.</p>
+    <p>a</p>
+    <p>b</p>
+    <p>c</p>
+    <p>d</p>
+    <p>e</p>
+    <p>f</p>
+    <p>g</p>
+    <p>h</p>
+    <p>i</p>
+</div>
+<script>
+test(function() {
+    if (!window.eventSender)
+        return;
+    var leftButton = 0;

[tdresser, 2016/09/08 18:23:38]

This appears unused.

+    var DragAndDropStart = 1552;
+    var scrollable = document.getElementById("scrollable");
+    var selStartX = scrollable.offsetLeft + 5;

[tdresser, 2016/09/08 18:23:38]

I'd just go with selectionStartX etc.

+    var selStartY = scrollable.offsetTop + 25;
+    var selEndX = selStartX + 50;
+    var selEndY = selStartY;
+    var dragStartX = selStartX;

[tdresser, 2016/09/08 18:23:38]

I'd move the drag co-ordinates after the first mouse up, as then it's clear that
they aren't used in the first set of mouse* calls.

+    var dragStartY = selStartY;
+    var dragEndX = dragStartX;
+    var dragEndY = scrollable.offsetTop + 90; 
+    eventSender.mouseMoveTo(selStartX, selStartY);
+    eventSender.mouseDown();
+    eventSender.mouseMoveTo(selEndX, selEndY);
+    eventSender.mouseUp();
+    eventSender.mouseMoveTo(dragStartX, dragStartY);
+    eventSender.mouseDown();
+    eventSender.mouseMoveTo(dragEndX, dragEndY);
+    eventSender.mouseUp();
+    assert_true(internals.isUseCounted(document, DragAndDropStart));
+}, "Tests that selection-drag autoscroll is recorded in UMA usecounter.");      
+</script>