Fix GCC 4.7 (C++11) compilation.

src/profile-generator.cc

Index: src/profile-generator.cc
diff --git a/src/profile-generator.cc b/src/profile-generator.cc
index c03e5261701d0e3f62318b6d7348615da507f2e6..da2a969e7123ea132e550cee99a0df60b7a0f208 100644
--- a/src/profile-generator.cc
+++ b/src/profile-generator.cc
@@ -3606,9 +3606,9 @@ void HeapSnapshotJSONSerializer::SerializeSnapshot() {
   writer_->AddString(",\"meta\":");
   // The object describing node serialization layout.
   // We use a set of macros to improve readability.
-#define JSON_A(s) "["s"]"
-#define JSON_O(s) "{"s"}"
-#define JSON_S(s) "\""s"\""
+#define JSON_A(s) "[" s "]"
+#define JSON_O(s) "{" s "}"
+#define JSON_S(s) "\"" s "\""
   writer_->AddString(JSON_O(
     JSON_S("node_fields") ":" JSON_A(
         JSON_S("type") ","