Implement Middle Click Autoscroll on all platforms not just Windows

third_party/WebKit/LayoutTests/fast/events/resources/middleClickAutoscroll.js

 var autoscrollInterval = 50;
 var middleButton = 1;
-var panScrollRadius = 15; // from FrameView::noPanScrollRadius
+var middleClickAutoscrollRadius = 15; // from FrameView::noPanScrollRadius
 
 window.jsTestIsAsync = true;
 
 function $(id)
 {
     return document.getElementById(id);
 }
 
 function testPanScroll(param)
 {
@@ skipping 52 matching lines @@
             testPassed('autoscroll stopped');
             finishTest();
         }, autoscrollInterval * 2);
     };
 
     if (!window.eventSender)
         return;
     var startX = param.startX || scrollable.offsetLeft + 5;
     var startY = param.startY || scrollable.offsetTop + 5;
     var endX = param.endX || scrollable.offsetLeft + 5;
-    var endY = param.endY || scrollable.offsetTop + panScrollRadius + 6;
+    var endY = param.endY || scrollable.offsetTop + middleClickAutoscrollRadius + 6;
     eventSender.mouseMoveTo(startX, startY);
     eventSender.mouseDown(middleButton);
     if (param.clickOrDrag == 'click')
         eventSender.mouseUp(middleButton);
     eventSender.mouseMoveTo(endX, endY);
 }