Add StorageThread::dispatch
https://bugs.webkit.org/show_bug.cgi?id=115308

Reviewed by Andreas Kling.

StorageThread::dispatch takes a Function<void ()> and runs it on the storage thread. This will be used to eventually get rid of StorageTask.

* storage/StorageTask.cpp:
(WebCore::StorageTask::StorageTask):
(WebCore::StorageTask::performTask):
* storage/StorageTask.h:
(WebCore::StorageTask::createDispatch):
* storage/StorageThread.cpp:
(WebCore::StorageThread::dispatch):
* storage/StorageThread.h:
(StorageThread):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149241 268f45cc-cd09-0410-ab3c-d52691b4dbfc