Implement Middle Click Autoscroll on all platforms not just Windows

third_party/WebKit/LayoutTests/fast/events/middleClickAutoscroll-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 middleClickAutoscrollRadius = 15;
+    var middleClickAutoscrollStart = 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;
+    var endY = scrollable.offsetTop + middleClickAutoscrollRadius + 6;
     eventSender.mouseMoveTo(startX, startY);
     eventSender.mouseDown(middleButton);
     eventSender.mouseMoveTo(endX, endY);
-    assert_true(internals.isUseCounted(document, PanScrollingStart));
+    assert_true(internals.isUseCounted(document, middleClickAutoscrollStart));
 })
 </script>