Add test case for mouseleave events within window bounds

LayoutTests/fast/events/event-sender-mouseleave.html

+<!DOCTYPE html>
+<style>
+#testBox {
+    background: blue;
+    top: 100px;
+    left: 100px;
+    width: 100px;
+    height: 100px;
+    position: absolute;
+}
+</style>
+<div id="testBox">
+</div>
+<p id="description">
+</p>
+<script src="../../resources/js-test.js"></script>
+<script>
+    window.jsTestIsAsync = true;
+
+    description('Test that a mouse leave event is triggered.');

[Rick Byers, 2015/03/06 16:05:37]

nit add: "when the OS tells us the mouse has left without changing position"

[Miyoung Shin(g), 2015/03/09 15:47:27]

Done.

+    var hasMouseLeave = false;
+    var hasMouseOut = false;
+    var box = document.getElementById('testBox');
+    box.addEventListener('mouseleave', function () {
+        hasMouseLeave = true;

[Rick Byers, 2015/03/06 16:05:37]

please verify here (and below if you like) that the mouse event co-ordinates are
still inside the box.  This worked fine in the past in the common case where the
co-ordinates were actually outside the box, but the bug was that if the
co-ordinates were still technically inside the box then we'd get only a
mousemove event - even though the OS is telling us the mouse isn't over our
window anymore.

eg. just:
 shouldBe('box.offsetLeft + 10', e.clientX)
 shouldBe('box.offsetTop + 10', e.clientY)
or something like that should be sufficient.

[Miyoung Shin(g), 2015/03/09 15:47:27]

Done. Thank you for your comment.

+    });
+    box.addEventListener('mouseout', function () {
+        hasMouseOut = true;
+    });
+    if (window.testRunner) {
+        eventSender.mouseMoveTo(1, 1);
+        eventSender.mouseMoveTo(box.offsetLeft + 10, box.offsetTop + 10);
+        eventSender.mouseLeave();
+        shouldBeTrue('hasMouseLeave');
+        shouldBeTrue('hasMouseOut');
+        finishJSTest();
+    }
+</script>