[ServiceWorker] CacheStorage tests.

LayoutTests/http/tests/serviceworker/resources/cache-storage-keys-worker.js

+importScripts('worker-test-harness.js');
+
+var test_cache_list =
+  ['', 'example', 'Another cache name', 'A', 'a', 'ex ample'];
+
+promise_test(function(test) {
+    return self.caches.keys()
+      .then(function(keys) {
+          assert_true(Array.isArray(keys),
+                      'CacheStorage.keys should return an Array.');
+          return Promise.all(keys.map(function(key) {
+              return self.caches.delete(key);
+            }));
+        })
+      .then(function() {
+          return Promise.all(test_cache_list.map(function(key) {
+              return self.caches.create(key);
+            }));
+        })
+
+      .then(function() { return self.caches.keys(); })
+      .then(function(keys) {
+          assert_true(Array.isArray(keys),
+                      'CacheStorage.keys should return an Array.');
+          assert_array_equals(keys,
+                              test_cache_list,
+                              'CacheStorage.keys should only return ' +
+                              'existing caches.');
+        });
+  }, 'CacheStorage keys');