Use StringBuilder::appendLiteral() / StringBuilder::append(char) when possible

Source/core/css/parser/BisonCSSParserTest.cpp

Index: Source/core/css/parser/BisonCSSParserTest.cpp
diff --git a/Source/core/css/parser/BisonCSSParserTest.cpp b/Source/core/css/parser/BisonCSSParserTest.cpp
index 1a8b3f085c7c25800807d9a8022165ce66d18629..e50068eb8c73e0facba244947831ec0f28c15a92 100644
--- a/Source/core/css/parser/BisonCSSParserTest.cpp
+++ b/Source/core/css/parser/BisonCSSParserTest.cpp
@@ -79,7 +79,7 @@ static void testMediaQuery(const char* expected, MediaQuerySet& querySet)
         ++i;
         if (i >= queryVectorSize)
             break;
-        output.append(", ");
+        output.appendLiteral(", ");
     }
     ASSERT_STREQ(expected, output.toString().ascii().data());
 }