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

LayoutTests/fast/css/sticky/sticky-as-positioning-container.html

+<style type="text/css">
+div {
+    height: 100px;
+    width: 100px;
+}
+.spacer {
+    background-color: red;
+}
+.sticky {
+    background-color: green;
+    position: sticky;
+}
+.absoffset {
+    background-color: green;
+    position: absolute;
+    top: -100px;
+}
+</style>
+<div class="spacer"></div>
+<div class="sticky">
+    <!--
+    Tests that the absolute positioned child is positioned relative to the sticky container. You should see only a
+    green box as this will cover up the red box above the sticky div.

[Rick Byers, 2015/07/03 01:43:55]

Prefer ref tests over pixel tests:
https://www.chromium.org/developers/testing/webkit-layout-tests/testexpectations.
 Pixel tests are hard to maintain in general (and use a lot more storage space -
which has been a problem for the repo merge!).

Or perhaps a layout test seems adequate in this case too (just use the
testRunner API to disable generating pixel results).

[flackr, 2015/07/14 15:24:35]

Done. Even though a layout test would have worked for this case I used a ref
test since I think it better describes the intended behavior.

+    -->
+    <div class="absoffset"></div>
+</div>