New parser mode for CSSOM @viewport descriptors.

Source/core/css/CSSParserMode.h

Index: Source/core/css/CSSParserMode.h
diff --git a/Source/core/css/CSSParserMode.h b/Source/core/css/CSSParserMode.h
index 59ef416d316a2fc4f12b27cd0b81a22f2186ccc9..37e44349aa5df47c4fd25e945af607ffa4b5b6ed 100644
--- a/Source/core/css/CSSParserMode.h
+++ b/Source/core/css/CSSParserMode.h
@@ -44,7 +44,9 @@ enum CSSParserMode {
     SVGAttributeMode,
     // User agent style sheet should always be in strict mode. Enables internal
     // only properties and values.
-    UASheetMode
+    UASheetMode,
+    // Parsing @viewport descriptors. Always strict.
+    ViewportMode
 };
 
 inline CSSParserMode strictToCSSParserMode(bool inStrictMode)
@@ -54,7 +56,7 @@ inline CSSParserMode strictToCSSParserMode(bool inStrictMode)
 
 inline bool isStrictParserMode(CSSParserMode cssParserMode)
 {
-    return cssParserMode == CSSStrictMode || cssParserMode == SVGAttributeMode || cssParserMode == UASheetMode;
+    return cssParserMode != CSSQuirksMode;
 }
 
 struct CSSParserContext {