Fix lifetime management issue in ResourceLoadStatisticsPersistentStorage::scheduleOrW...

Fix lifetime management issue in ResourceLoadStatisticsPersistentStorage::scheduleOrWriteMemoryStore()
https://bugs.webkit.org/show_bug.cgi?id=174790

Reviewed by Brady Eidson.

Fix lifetime management issue in ResourceLoadStatisticsPersistentStorage::scheduleOrWriteMemoryStore().
WorkQueue::dispatchAfter() keeps the WorkQueue alive because its implementation keeps a strong ref to
|this|. As a result, the lambda passed to dispatchAfter(), which calls writeMemoryStoreToDisk(), can
get executed after the store is gone.

* UIProcess/Storage/ResourceLoadStatisticsPersistentStorage.cpp:
(WebKit::ResourceLoadStatisticsPersistentStorage::ResourceLoadStatisticsPersistentStorage):
(WebKit::ResourceLoadStatisticsPersistentStorage::scheduleOrWriteMemoryStore):
* UIProcess/Storage/ResourceLoadStatisticsPersistentStorage.h:
(WebKit::ResourceLoadStatisticsPersistentStorage::createWeakPtr):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@219828 268f45cc-cd09-0410-ab3c-d52691b4dbfc