Upstream service worker prototype test to WPT

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/immutable-prototype-serviceworker.js

 function prototypeChain(global) {
   let result = [];
   while (global !== null) {
     let thrown = false;
     let next = global.__proto__;
     try {
       global.__proto__ = {};
       result.push('mutable');
     } catch (e) {
       result.push('immutable');
     }
     global = next;
   }
   return result;
 }
 
 self.onmessage = function(e) {
   e.data.postMessage(prototypeChain(self));
 };