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

Source/core/layout/LayoutBlockFlow.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 2757 matching lines @@
     for (curr = firstChildBox(); curr && curr->selectionState() == SelectionNone; curr = curr->nextSiblingBox()) { }
 
     for (bool sawSelectionEnd = false; curr && !sawSelectionEnd; curr = curr->nextSiblingBox()) {
         SelectionState childState = curr->selectionState();
         if (childState == SelectionBoth || childState == SelectionEnd)
             sawSelectionEnd = true;
 
         if (curr->isFloatingOrOutOfFlowPositioned())
             continue; // We must be a normal flow object in order to even be considered.
 
-        if (curr->isRelPositioned() && curr->hasLayer()) {
+        if (curr->isInFlowPositioned() && curr->hasLayer()) {
             // If the relposition offset is anything other than 0, then treat this just like an absolute positioned element.
             // Just disregard it completely.
             LayoutSize relOffset = curr->layer()->offsetForInFlowPosition();
             if (relOffset.width() || relOffset.height())
                 continue;
         }
 
         bool paintsOwnSelection = curr->shouldPaintSelectionGaps() || curr->isTable(); // FIXME: Eventually we won't special-case table like this.
         bool fillBlockGaps = paintsOwnSelection || (curr->canBeSelectionLeaf() && childState != SelectionNone);
         if (fillBlockGaps) {
@@ skipping 255 matching lines @@
     FrameView* frameView = document().view();
     LayoutUnit top = (style()->position() == FixedPosition) ? 0 : frameView->scrollOffset().height();
     int visibleHeight = frameView->visibleContentRect(IncludeScrollbars).height();
     if (size().height() < visibleHeight)
         top += (visibleHeight - size().height()) / 2;
     setY(top);
     dialog->setCentered(top);
 }
 
 } // namespace blink