Fix position:sticky location bug when scroller is not a stacking context

third_party/WebKit/LayoutTests/compositing/overflow/composited-sticky-element-stacking-context-expected.html

 <script>
 onload = function() {
   scroller.scrollTop = 100;
 }
 </script>
 <style>
 .composited {
+  /* Triggers promotion and creates a stacking context */
   will-change: transform;
 }
 
 #scroller {
   background: white;
   height: 200px;
   overflow: auto;
   width: 200px;
 }
 .sticky {
   position: relative;
   top: 100px;
   height: 50px;
   background: green;
 }
 .spacer {
   height: 2000px;
 }
 </style>
 <div id="scroller" class="composited">
   <div class="composited sticky"></div>
   <div class="spacer"></div>
 </div>