Add support for new CSS ::slotted() pseudo element

third_party/WebKit/Source/core/css/resolver/SharedStyleFinder.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 260 matching lines @@
     ShadowRoot* root1 = element().containingShadowRoot();
     ShadowRoot* root2 = candidate.containingShadowRoot();
     if (root1 && root2 && root1->type() != root2->type())
         return false;
 
     if (document().containsValidityStyleRules()) {
         if (candidate.isValidElement() != element().isValidElement())
             return false;
     }
 
+    // TODO(kochi): Investigate cases when assigned slots can share styles.
+    if (element().assignedSlot())

[kochi, 2016/01/07 09:54:09]

I found that this now causes assertion in ElementShadow::assignedSlotFor(),
as if ElementShadow is not for V1 shadow, ElementShadow does not have
slot assignment information.

A workaround is to change this to call assignedSlotForBinding(), but IMHO
we should have an internal API for checking if an element has any assigned
slot.

+        return false;
+
     return true;
 }
 
 bool SharedStyleFinder::documentContainsValidCandidate() const
 {
     for (Element& element : ElementTraversal::startsAt(document().documentElement())) {
         if (element.supportsStyleSharing() && canShareStyleWithElement(element))
             return true;
     }
     return false;
@@ skipping 58 matching lines @@
     // Tracking child index requires unique style for each node. This may get set by the sibling rule match above.
     if (!element().parentElementOrShadowRoot()->childrenSupportStyleSharing()) {
         INCREMENT_STYLE_STATS_COUNTER(*m_styleResolver, sharedStyleRejectedByParent, 1);
         return nullptr;
     }
 
     return shareElement->mutableComputedStyle();
 }
 
 }