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 075d2e28cee8decb30770cc301c50c45b906cc30..c6e796d1111b303b0b2741a7296a57ab1f74e24d 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

[eseidel, 2013/09/17 22:26:00]

If you're willing to go one more round here, it would be nice to have a comment
explaining what is different aboug this mode than others.  If it's just the
"width" thing, that seems easy to put in a comment.  If its complicated, we
could link to the spec.

[rune, 2013/09/18 08:13:09]

The main thing about this mode is to use viewport descriptor parsing rules
instead of style declaration parsing rules. While parsing a stylesheet, this is
triggered by setting inViewport() when the rule header (@viewport) is
encountered. This context is (or was) lost in the StylePropertySet, so modifying
the StylePropertySet through CSSOM would use the parsing rules for style
declarations instead of viewport descriptors.

I've extended/added comments.

 };
 
 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 {