Use an std::function for filtering session state data

Use an std::function for filtering session state data
https://bugs.webkit.org/show_bug.cgi?id=134481

Reviewed by Sam Weinig.

* UIProcess/API/C/WKPage.cpp:
(WKPageCopySessionState):
* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView _sessionState]):
* UIProcess/WebBackForwardList.h:
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::sessionStateData):
* UIProcess/WebPageProxy.h:
* UIProcess/cf/WebBackForwardListCF.cpp:
(WebKit::WebBackForwardList::createCFDictionaryRepresentation):
* UIProcess/cf/WebPageProxyCF.cpp:
(WebKit::WebPageProxy::sessionStateData):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@170644 268f45cc-cd09-0410-ab3c-d52691b4dbfc