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

LayoutTests/fast/css/sticky/resources/parsing-position-sticky.js

Index: LayoutTests/fast/css/sticky/resources/parsing-position-sticky.js
diff --git a/LayoutTests/fast/css/sticky/resources/parsing-position-sticky.js b/LayoutTests/fast/css/sticky/resources/parsing-position-sticky.js
new file mode 100644
index 0000000000000000000000000000000000000000..1f59ccc5b97875413bd55365e04341a849f916ad
--- /dev/null
+++ b/LayoutTests/fast/css/sticky/resources/parsing-position-sticky.js
@@ -0,0 +1,14 @@
+description("This tests checks that the sticky value for position parses correctly.");
+
+function test(value)
+{
+    var div = document.createElement("div");
+    div.setAttribute("style", value);
+    document.body.appendChild(div);
+
+    var result = div.style.getPropertyValue("position");
+    document.body.removeChild(div);
+    return result;
+}
+
+shouldBe('test("position: sticky;")', '"sticky"');