Expose Cache Storage API in global window/worker scope

LayoutTests/http/tests/cachestorage/script-tests/cache-storage-keys.js

-importScripts('worker-testharness.js');
-importScripts('../../resources/testharness-helpers.js');
-importScripts('override_assert_object_equals.js');
+if (self.importScripts) {
+    importScripts('/resources/testharness.js');
+    importScripts('/resources/testharness-helpers.js');
+    importScripts('../resources/test-helpers.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.open(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');
+
+done();