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

Source/core/css/CSSParser-in.cpp

Index: Source/core/css/CSSParser-in.cpp
diff --git a/Source/core/css/CSSParser-in.cpp b/Source/core/css/CSSParser-in.cpp
index 500e47197112ea7a11162a52b5e25b710b76b917..381a6da90f1da0c47c70adabdee3b70086b80fe7 100644
--- a/Source/core/css/CSSParser-in.cpp
+++ b/Source/core/css/CSSParser-in.cpp
@@ -1717,7 +1717,7 @@ bool CSSParser::parseValue(CSSPropertyID propId, bool important)
     }
 
     if (inViewport()) {
-        if (!RuntimeEnabledFeatures::cssViewportEnabled())
+        if (!RuntimeEnabledFeatures::cssViewportEnabled() && m_context.mode != UASheetMode)

[kenneth.r.christiansen, 2013/09/02 09:10:49]

Add comment: // Allow @viewport rules from UA stylesheets even if the feature is
disabled

[rune, 2013/09/02 11:53:39]

Done.

             return false;
 
         return parseViewportProperty(propId, important);
@@ -11634,7 +11634,8 @@ unsigned CSSParser::safeUserStringTokenOffset()
 
 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)
         return 0;
 
     m_allowImportRules = m_allowNamespaceDeclarations = false;
@@ -11653,7 +11654,7 @@ StyleRuleBase* CSSParser::createViewportRule()
 
 bool CSSParser::parseViewportProperty(CSSPropertyID propId, bool important)
 {
-    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled());
+    ASSERT(RuntimeEnabledFeatures::cssViewportEnabled() || m_context.mode == UASheetMode);
 
     CSSParserValue* value = m_valueList->current();
     if (!value)
@@ -11713,7 +11714,7 @@ bool CSSParser::parseViewportProperty(CSSPropertyID propId, bool important)
 
 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)