Populates sourceDevice attribute into MouseEvent

LayoutTests/fast/events/uievent-with-inputdevice.html

 <script src="../../resources/js-test.js"></script>
 <script>
     description("This tests that UIEvent and its subclass will have sourceDevice set to be null by default, and it can also be passed when initialization.")
 
+    // Creating UIEvent.
     var uievent = document.createEvent('UIEvent');
     shouldBeNonNull("uievent");
     shouldBeNull("uievent.sourceDevice");  
     
     uievent = new UIEvent('eventType', { sourceDevice: new InputDevice({ firesTouchEvents: false }) });
     shouldBeNonNull("uievent.sourceDevice");
     shouldBeFalse("uievent.sourceDevice.firesTouchEvents");
     
     uievent = new UIEvent('eventType', { sourceDevice: new InputDevice({ firesTouchEvents: true }) });
     shouldBeNonNull("uievent.sourceDevice");
     shouldBeTrue("uievent.sourceDevice.firesTouchEvents");
     
+    // Creating TouchEvent.
     var touchevent = document.createEvent("TouchEvent");
     shouldBeNonNull("touchevent");
     shouldBeNull("touchevent.sourceDevice");
+    
+    // Creating MouseEvent.
+    document.addEventListener("mousedown", function(event) {
+      shouldBeNonNull("event");
+      shouldBeNull("event.sourceDevice");
+    });
 
-</script>
+    var e = new MouseEvent('mousedown');
+    document.dispatchEvent(e);
+    
+    var sourceDevice = new InputDevice({ firesTouchEvents: false });
+    shouldBeFalse("sourceDevice.firesTouchEvents");
+    
+    var mouseevent = document.createEvent("MouseEvents");
+    mouseevent.initMouseEvent("mousedown", true, true, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null, sourceDevice);

[Rick Byers, 2015/06/23 00:55:57]

nit: please use the 'new MouseEvent' style instead or as well (just like the
'new UIEvent' above).  If we're only going to test one of the patterns than the
modern style is probably the one we really care about.

[lanwei, 2015/06/23 22:19:20]

Done.

+    shouldBeNonNull("mouseevent.sourceDevice");
+    shouldBeFalse("mouseevent.sourceDevice.firesTouchEvents");
+
+</script>