[POC-WIP] Link window focus tokens with ExecutionContext.

LayoutTests/http/tests/serviceworker/resources/client-focus.js

Index: LayoutTests/http/tests/serviceworker/resources/client-focus.js
diff --git a/LayoutTests/http/tests/serviceworker/resources/client-focus.js b/LayoutTests/http/tests/serviceworker/resources/client-focus.js
new file mode 100644
index 0000000000000000000000000000000000000000..27a48443f55870a08edf56bd65ca7c5a7f8a1246
--- /dev/null
+++ b/LayoutTests/http/tests/serviceworker/resources/client-focus.js
@@ -0,0 +1,12 @@
+self.onmessage = function(e) {
+  self.clients.getAll().then(function(clients) {
+    clients.forEach(function(client) {
+      if (('focus' in client) && (typeof(client.focus) == 'function'))
+        client.postMessage('focus() is present');
+      client.focus().then(function(result) {
+        client.postMessage('focus() succeeded with ' + result);
+        client.postMessage('quit');
+      });
+    });
+  });
+}