[inspector] provide more usefull error message for non serializable value

src/inspector/StringUtil.h

Index: src/inspector/StringUtil.h
diff --git a/src/inspector/StringUtil.h b/src/inspector/StringUtil.h
index 3a69454d95ee3d077275ba4a338ec66cac8a488b..63ffebbcce5263748f092815b0661f381303558b 100644
--- a/src/inspector/StringUtil.h
+++ b/src/inspector/StringUtil.h
@@ -37,7 +37,8 @@ std::unique_ptr<protocol::Value> parseJSON(const String16& json);
 
 }  // namespace protocol
 
-std::unique_ptr<protocol::Value> toProtocolValue(v8::Local<v8::Context>,
+std::unique_ptr<protocol::Value> toProtocolValue(protocol::String* errorString,
+                                                 v8::Local<v8::Context>,
                                                  v8::Local<v8::Value>,
                                                  int maxDepth = 1000);