Adapt StringBuilder's append and toString methods via StringUtil helper.

src/inspector/string-util.h

Index: src/inspector/string-util.h
diff --git a/src/inspector/string-util.h b/src/inspector/string-util.h
index c484aab2edf6326a44953d02d32de26149ae7523..1936c11d0061c75bdf4867c007d61bb28f015ece 100644
--- a/src/inspector/string-util.h
+++ b/src/inspector/string-util.h
@@ -33,9 +33,21 @@ class StringUtil {
   }
   static String fromDouble(double number) { return String::fromDouble(number); }
   static const size_t kNotFound = String::kNotFound;
+  static void builderAppend(StringBuilder& builder, const String& s) {
+    builder.append(s);
+  }
+  static void builderAppend(StringBuilder& builder, UChar c) {
+    builder.append(c);
+  }
+  static void builderAppend(StringBuilder& builder, const char* s, size_t len) {
+    builder.append(s, len);
+  }
   static void builderReserve(StringBuilder& builder, size_t capacity) {
     builder.reserveCapacity(capacity);
   }
+  static String builderToString(StringBuilder& builder) {
+    return builder.toString();
+  }
   static std::unique_ptr<protocol::Value> parseJSON(const String16& json);
   static std::unique_ptr<protocol::Value> parseJSON(const StringView& json);
 };