Get rid of more uses of StorageTask
https://bugs.webkit.org/show_bug.cgi?id=115311

Reviewed by Dean Jackson.

Make StorageAreaSync derive from ThreadSafeRefCounted as well, since it's passed between threads.

* storage/StorageAreaSync.h:
* storage/StorageSyncManager.cpp:
(WebCore::StorageSyncManager::scheduleImport):
(WebCore::StorageSyncManager::scheduleSync):
(WebCore::StorageSyncManager::scheduleDeleteEmptyDatabase):
* storage/StorageTask.cpp:
(WebCore::StorageTask::StorageTask):
(WebCore::StorageTask::performTask):
* storage/StorageTask.h:
(StorageTask):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149244 268f45cc-cd09-0410-ab3c-d52691b4dbfc