Add diagnostic for nested @-rules that are ignored. (Bug 652014) r=bzbarsky

dom/locales/en-US/chrome/layout/css.properties

@@ -55,6 +55,7 @@ PEImportNotURI=Expected URI in @import rule but found '%1$S'.
 PEImportBadURI=Invalid URI in @import rule: '%1$S'.
 PEImportUnexpected=Found unexpected '%1$S' within @import.
 PEGroupRuleEOF=end of @media or @-moz-document rule
+PEGroupRuleNestedAtRule=%1$S rule not allowed within @media or @-moz-document rule.
 PEMozDocRuleBadFunc=Expected url(), url-prefix(), or domain() in @-moz-document rule but found '%1$S'.
 PEMozDocRuleNotURI=Expected URI in @-moz-document rule but found '%1$S'.
 PEAtNSPrefixEOF=namespace prefix in @namespace rule

layout/style/nsCSSParser.cpp

@@ -2064,6 +2064,8 @@ CSSParserImpl::ParseGroupRule(css::GroupRule* aRule,
       break;
     }
     if (eCSSToken_AtKeyword == mToken.mType) {
+      REPORT_UNEXPECTED_TOKEN(PEGroupRuleNestedAtRule);
+      OUTPUT_ERROR();
       SkipAtRule(PR_TRUE); // group rules cannot contain @rules
       continue;
     }