Add position: sticky as supported position value when CSSStickyPosition is enabled.

LayoutTests/fast/css/sticky/sticky-stacking-context.html

Index: LayoutTests/fast/css/sticky/sticky-stacking-context.html
diff --git a/LayoutTests/fast/css/sticky/sticky-stacking-context.html b/LayoutTests/fast/css/sticky/sticky-stacking-context.html
new file mode 100644
index 0000000000000000000000000000000000000000..aea9b85481ee15cc2a309a3a813297376d529dd7
--- /dev/null
+++ b/LayoutTests/fast/css/sticky/sticky-stacking-context.html
@@ -0,0 +1,20 @@
+<style type="text/css">
+div {
+    height: 100px;
+    width: 100px;
+}
+.sticky {
+    background-color: green;
+    position: sticky;
+}
+.red {
+    background-color: red;
+    margin-top: -100px;
+}
+</style>
+<!--
+Tests that the sticky node will be popped out into a stacking context on top of the node which follows it. You should
+see no red boxes as the green sticky div should pop on top of the red div placed under it.
+-->
+<div class="sticky"></div>
+<div class="red"></div>