Use css-device-adapt constraining for legacy viewport tags.

Source/core/css/CSSParser-in.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 1662 matching lines @@
 
     if (!m_valueList)
         return false;
 
     CSSParserValue* value = m_valueList->current();
 
     if (!value)
         return false;
 
     if (inViewport()) {
-        if (!RuntimeEnabledFeatures::cssViewportEnabled())
+        // Allow @viewport rules from UA stylesheets even if the feature is disabled.
+        if (!RuntimeEnabledFeatures::cssViewportEnabled() && m_context.mode != UASheetMode)
             return false;
 
         return parseViewportProperty(propId, important);
     }
 
     // Note: m_parsedCalculation is used to pass the calc value to validUnit and then cleared at the end of this function.
     // FIXME: This is to avoid having to pass parsedCalc to all validUnit callers.
     ASSERT(!m_parsedCalculation);
 
     CSSValueID id = value->id;
@@ skipping 9933 matching lines @@
         m_sourceDataHandler->startEndUnknownRule();
 }
 
 unsigned CSSParser::safeUserStringTokenOffset()
 {
     return min(tokenStartOffset(), static_cast<unsigned>(m_length - 1 - m_parsedTextSuffixLength)) - m_parsedTextPrefixLength;
 }
 
 StyleRuleBase* CSSParser::createViewportRule()
 {
-    if (!RuntimeEnabledFeatures::cssViewportEnabled()) {
+    // Allow @viewport rules from UA stylesheets even if the feature is disabled.
+    if (!RuntimeEnabledFeatures::cssViewportEnabled() && m_context.mode != UASheetMode) {
         endRuleBody(true);
         return 0;
     }
 
     m_allowImportRules = m_allowNamespaceDeclarations = false;
 
     RefPtr<StyleRuleViewport> rule = StyleRuleViewport::create();
 
     rule->setProperties(createStylePropertySet());
     clearProperties();
 
     StyleRuleViewport* result = rule.get();
     m_parsedRules.append(rule.release());
     endRuleBody();
 
     return result;
 }
 
 bool CSSParser::parseViewportProperty(CSSPropertyID propId, bool important)
 {
-    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled());
+    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled() || m_context.mode == UASheetMode);
 
     CSSParserValue* value = m_valueList->current();
     if (!value)
         return false;
 
     CSSValueID id = value->id;
     bool validPrimitive = false;
 
     switch (propId) {
     case CSSPropertyMinWidth: // auto | extend-to-zoom | <length> | <percentage>
@@ skipping 39 matching lines @@
             addProperty(propId, parsedValue.release(), important);
             return true;
         }
     }
 
     return false;
 }
 
 bool CSSParser::parseViewportShorthand(CSSPropertyID propId, CSSPropertyID first, CSSPropertyID second, bool important)
 {
-    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled());
+    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled() || m_context.mode == UASheetMode);
     unsigned numValues = m_valueList->size();
 
     if (numValues > 2)
         return false;
 
     ShorthandScope scope(this, propId);
 
     if (!parseViewportProperty(first, important))
         return false;
 
@@ skipping 244 matching lines @@
 {
     // The tokenizer checks for the construct of an+b.
     // However, since the {ident} rule precedes the {nth} rule, some of those
     // tokens are identified as string literal. Furthermore we need to accept
     // "odd" and "even" which does not match to an+b.
     return equalIgnoringCase(token, "odd") || equalIgnoringCase(token, "even")
         || equalIgnoringCase(token, "n") || equalIgnoringCase(token, "-n");
 }
 
 }