Oilpan: fix build after r193243.

Source/core/css/parser/CSSParserImpl.cpp

Index: Source/core/css/parser/CSSParserImpl.cpp
diff --git a/Source/core/css/parser/CSSParserImpl.cpp b/Source/core/css/parser/CSSParserImpl.cpp
index 5f3bcc1d45be32406d74c6f583177f11dc94e5da..f381f1739cf1f8cb5874d39c84513cd6329808b4 100644
--- a/Source/core/css/parser/CSSParserImpl.cpp
+++ b/Source/core/css/parser/CSSParserImpl.cpp
@@ -520,7 +520,7 @@ void CSSParserImpl::consumeDeclarationList(CSSParserTokenRange range, StyleRule:
         }
         case AtKeywordToken: {
             RefPtrWillBeRawPtr<StyleRuleBase> rule = consumeAtRule(range, NoRules);
-            ASSERT(!rule);
+            ASSERT_UNUSED(rule, !rule);
             break;
         }
         default: // Parse error, unexpected token in declaration list