Remove HistoryController class members to track loads in progress.

They can easily be stack variables and only used as needed. This patch also
removes HistoryController::inSameDocumentLoad(), which shouldn't be necessary
anymore.

BUG=none

Review URL: https://codereview.chromium.org/126603003

git-svn-id: svn://svn.chromium.org/blink/trunk@164641 bbb929c8-8fbe-4397-9dbb-9b2b20218538