function storageForId() should be used in _domStorageItemUpdated instead of DOMStorageModel._storages[]

The commit with commit hash: "17af239b44a9a02b2d6b02f9f5aaaf6cb7b4ce1f" 
(https://bugs.webkit.org/show_bug.cgi?id=109572) missed to use the function storageForId
instead of DOMStorageModel._storages[] due to which the update is not triggered.

R=apavlov

Review URL: https://chromiumcodereview.appspot.com/22396002

git-svn-id: svn://svn.chromium.org/blink/trunk@155616 bbb929c8-8fbe-4397-9dbb-9b2b20218538