may be called before implementations of BackForwardControllerClient have
access to a Page.

Reviewed by Anders Carlsson.

WebCore: 

* history/BackForwardController.cpp:
(WebCore::BackForwardController::BackForwardController):
* history/BackForwardControllerClient.h:

WebKit2: 

* WebProcess/WebCoreSupport/WebBackForwardControllerClient.cpp:
(WebKit::WebBackForwardControllerClient::createBackForwardList):
* WebProcess/WebCoreSupport/WebBackForwardControllerClient.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@62804 268f45cc-cd09-0410-ab3c-d52691b4dbfc