[d8] Use ValueSerializer for postMessage (instead of ad-hoc serializer)

test/mjsunit/d8-worker.js

Index: test/mjsunit/d8-worker.js
diff --git a/test/mjsunit/d8-worker.js b/test/mjsunit/d8-worker.js
index f172fb160338338476473503e54514cb1a994bde..2ed3b88eb5e55d1dbe531552d841a3204e9ec855 100644
--- a/test/mjsunit/d8-worker.js
+++ b/test/mjsunit/d8-worker.js
@@ -61,8 +61,8 @@ var workerScript =
          }
          break;
        case 7:
-         if (JSON.stringify(m) !== \"{'a':1,'b':2.5,'c':'three'}\")
-           throw new Error('Object');
+         if (JSON.stringify(m) !== '{"a":1,"b":2.5,"c":"three"}')
+           throw new Error('Object' + JSON.stringify(m));
          break;
        case 8:
          var ab = m;
@@ -88,7 +88,6 @@ var workerScript =
      }
    };`;
 
-
 if (this.Worker) {
   function createArrayBuffer(byteLength) {
     var ab = new ArrayBuffer(byteLength);