Restrict __qem usage to UA sheets

Source/core/css/parser/CSSPropertyParser.cpp

 /*
  * Copyright (C) 2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2012 Adobe Systems Incorporated. All rights reserved.
  * Copyright (C) 2012 Intel Corporation. All rights reserved.
  *
@@ skipping 229 matching lines @@
             value->unit = (unitflags & FLength) ? CSSPrimitiveValue::CSS_PX : CSSPrimitiveValue::CSS_DEG;
             return true;
         }
         if ((unitflags & FInteger) && value->isInt)
             return true;
         if ((unitflags & FPositiveInteger) && value->isInt && value->fValue > 0)
             return true;
         return false;
     case CSSPrimitiveValue::CSS_PERCENTAGE:
         return unitflags & FPercent;
-    // TODO(timloh): Restrict usage of __qem to UA sheets
     case CSSParserValue::Q_EMS:
+        if (cssParserMode != UASheetMode)
+            return false;
+    /* fallthrough intentional */
     case CSSPrimitiveValue::CSS_EMS:
     case CSSPrimitiveValue::CSS_REMS:
     case CSSPrimitiveValue::CSS_CHS:
     case CSSPrimitiveValue::CSS_EXS:
     case CSSPrimitiveValue::CSS_PX:
     case CSSPrimitiveValue::CSS_CM:
     case CSSPrimitiveValue::CSS_MM:
     case CSSPrimitiveValue::CSS_IN:
     case CSSPrimitiveValue::CSS_PT:
     case CSSPrimitiveValue::CSS_PC:
@@ skipping 8167 matching lines @@
         }
     }
 
     if (!list->length())
         return nullptr;
 
     return list.release();
 }
 
 } // namespace blink