Add Prerender type to NavigationType for nav timing 2

third_party/WebKit/Source/core/timing/PerformanceNavigation.idl

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 12 matching lines @@
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // https://w3c.github.io/navigation-timing/#the-performancenavigation-interface
 
+// Legacy support for NT1(https://www.w3.org/TR/navigation-timing/).
+
 [
     Exposed=Window,
 ] interface PerformanceNavigation {
     const unsigned short TYPE_NAVIGATE = 0;
     const unsigned short TYPE_RELOAD = 1;
     const unsigned short TYPE_BACK_FORWARD = 2;
     const unsigned short TYPE_RESERVED = 255;
     readonly attribute unsigned short type;
     readonly attribute unsigned short redirectCount;
     serializer = {attribute};
 };