Fix assert when interpolating using unset

Source/core/css/resolver/StyleBuilderCustom.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  * 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.
@@ skipping 103 matching lines @@
     bool isInitial = value->isInitialValue() || (!state.parentNode() && value->isInheritedValue());
 
     ASSERT(!isInherit || !isInitial); // isInherit -> !isInitial && isInitial -> !isInherit
     ASSERT(!isInherit || (state.parentNode() && state.parentStyle())); // isInherit -> (state.parentNode() && state.parentStyle())
 
     if (!state.applyPropertyToRegularStyle() && (!state.applyPropertyToVisitedLinkStyle() || !isValidVisitedLinkProperty(id))) {
         // Limit the properties that can be applied to only the ones honored by :visited.
         return;
     }
 
-    if (isInherit && !state.parentStyle()->hasExplicitlyInheritedProperties() && !CSSPropertyMetadata::isInheritedProperty(id))
+    if (isInherit && !state.parentStyle()->hasExplicitlyInheritedProperties() && !CSSPropertyMetadata::isInheritedProperty(id)) {
         state.parentStyle()->setHasExplicitlyInheritedProperties();
+    } else if (value->isUnsetValue()) {
+        ASSERT(!isInherit && !isInitial);
+        if (CSSPropertyMetadata::isInheritedProperty(id))
+            isInherit = true;
+        else
+            isInitial = true;
+    }
 
     StyleBuilder::applyProperty(id, state, value, isInitial, isInherit);
 }
 
 void StyleBuilderFunctions::applyInitialCSSPropertyColor(StyleResolverState& state)
 {
     Color color = RenderStyle::initialColor();
     if (state.applyPropertyToRegularStyle())
         state.style()->setColor(color);
     if (state.applyPropertyToVisitedLinkStyle())
@@ skipping 834 matching lines @@
         return;
     case CSSValueSuper:
         svgStyle.setBaselineShift(BS_SUPER);
         return;
     default:
         ASSERT_NOT_REACHED();
     }
 }
 
 } // namespace blink