Avoid calculating style for ::before/::after without content.

Source/core/css/ElementRuleCollector.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 281 matching lines @@
         if (m_sameOriginOnly && !ruleData.hasDocumentSecurityOrigin())
             return;
 
         PseudoId dynamicPseudo = result.dynamicPseudo;
         // If we're matching normal rules, set a pseudo bit if
         // we really just matched a pseudo-element.
         if (dynamicPseudo != NOPSEUDO && m_pseudoStyleRequest.pseudoId == NOPSEUDO) {
             if (m_mode == SelectorChecker::CollectingCSSRules || m_mode == SelectorChecker::CollectingStyleRules)
                 return;
             // FIXME: Matching should not modify the style directly.
-            if (m_style && dynamicPseudo < FIRST_INTERNAL_PSEUDOID)
-                m_style->setHasPseudoStyle(dynamicPseudo);
+            if (!m_style || dynamicPseudo >= FIRST_INTERNAL_PSEUDOID)
+                return;
+            if ((dynamicPseudo == BEFORE || dynamicPseudo == AFTER) && !ruleData.rule()->properties().hasProperty(CSSPropertyContent))
+                return;
+            m_style->setHasPseudoStyle(dynamicPseudo);
         } else {
             // Update our first/last rule indices in the matched rules array.
             ++ruleRange.lastRuleIndex;
             if (ruleRange.firstRuleIndex == -1)
                 ruleRange.firstRuleIndex = ruleRange.lastRuleIndex;
 
             // Add this rule to our list of matched rules.
             addMatchedRule(&ruleData, result.specificity, cascadeScope, cascadeOrder, matchRequest.styleSheetIndex, matchRequest.styleSheet);
             return;
         }
@@ skipping 29 matching lines @@
     // information about "scope".
     int firstRuleIndex = -1, lastRuleIndex = -1;
     RuleRange ruleRange(firstRuleIndex, lastRuleIndex);
     // FIXME: Verify whether it's ok to ignore CascadeScope here.
     collectMatchingRules(MatchRequest(ruleSet), ruleRange, SelectorChecker::DefaultBehavior);
 
     return m_matchedRules && !m_matchedRules->isEmpty();
 }
 
 } // namespace blink