Import w3c tests for the service workers

third_party/WebKit/LayoutTests/imported/wpt/service-workers/service-worker/resources/load_worker.js

+self.onmessage = function (evt) {
+    if (evt.data == "xhr") {
+        var xhr = new XMLHttpRequest();
+        xhr.open("GET", "synthesized-response.txt", true);
+        xhr.responseType = "text";
+        xhr.send();
+        xhr.onload = function (evt) {
+            postMessage(xhr.responseText);
+        };
+        xhr.onerror = function() {
+            postMessage("XHR failed!");
+        };
+    } else if (evt.data == "fetch") {
+        fetch("synthesized-response.txt")
+          .then(function(response) {
+              return response.text();
+            })
+          .then(function(data) {
+              postMessage(data);
+            })
+          .catch(function(error) {
+              postMessage("Fetch failed!");
+            });
+    } else if (evt.data == "importScripts") {
+        importScripts("synthesized-response.js");
+    } else {
+        throw "Unexpected message! " + evt.data;
+    }
+};