Expose Cache Storage API in global window/worker scope

LayoutTests/http/tests/cachestorage/resources/common-worker.js

+self.onmessage = function(e) {
+    var cache_name = e.data.name;
+
+    self.caches.open(cache_name)
+        .then(function(cache) {
+            return Promise.all([
+                cache.put('https://example.com/a', new Response('a')),
+                cache.put('https://example.com/b', new Response('b')),
+                cache.put('https://example.com/c', new Response('c'))
+            ]);
+        })
+        .then(function() {
+            self.postMessage('ok');
+        });
+};