Add ServiceWorker InstallPhaseEvent.waitUntil() layout test.

LayoutTests/http/tests/serviceworker/resources/install-phase-event-waituntil.js

+var pendingPorts = [];
+var portResolves = [];
+
+onmessage = function(e) {
+    var message = e.data;
+    if ('port' in message) {
+        var resolve = self.portResolves.shift();
+        if (resolve)
+            resolve(message.port);
+        else
+            self.pendingPorts.push(message.port);
+    }
+};
+
+function fulfillPromise() {
+    return new Promise(function(resolve) {
+        // Make sure the oninstall/onactivate callback returns first.
+        Promise.resolve().then(function() {
+            var port = self.pendingPorts.shift();
+            if (port)
+                resolve(port);
+            else
+                self.portResolves.push(resolve);
+        });
+    }).then(function(port) {
+        port.postMessage('SYNC');
+        return new Promise(function(resolve) {
+            port.onmessage = function(e) {
+                if (e.data == 'ACK')
+                    resolve();
+            };
+        });
+    });
+}
+
+function rejectPromise() {
+    return new Promise(function(resolve, reject) {
+        // Make sure the oninstall/onactivate callback returns first.
+        Promise.resolve().then(reject);
+    });
+}
+
+function stripScopeName(scope) {
+    return scope.split('/').slice(-1)[0];
+}
+
+oninstall = function(e) {
+    switch (stripScopeName(self.scope)) {
+        case 'install-fulfilled':
+            e.waitUntil(fulfillPromise());
+            break;
+        case 'install-rejected':
+            e.waitUntil(rejectPromise());
+            break;
+    }
+};
+
+onactivate = function(e) {
+    switch (stripScopeName(self.scope)) {
+        case 'activate-fulfilled':
+            e.waitUntil(fulfillPromise());
+            break;
+        case 'activate-rejected':
+            e.waitUntil(rejectPromise());
+            break;
+        case 'activate-multiple-fulfilled':
+            e.waitUntil(fulfillPromise());
+            e.waitUntil(fulfillPromise());
+            break;
+        case 'activate-reject-precedence':
+            e.waitUntil(fulfillPromise());
+            e.waitUntil(rejectPromise());
+            break;
+    }
+};