Don't leak WebHistoryService::Request on profile shutdown.

As the profile is shutting down, all URLRequests from that profile must be
destroyed. Instead of having HistoryService leak WebHistoryService's
requests, WebHistoryService maintains expiration requests internally. Those
don't get cancelled unless browser shutdown forces it to happen.

BUG=342164

Review URL: https://codereview.chromium.org/164703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252726 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252726