Cache Storage API tests: Fix WPT test bugs, remove redundant local copies

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

-if (self.importScripts) {
-    importScripts('/resources/testharness.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();