Remove most references to HistoryController in Source/core/.

Move saveDocumentAndScrollState and clearScrollPositionAndViewState() to FrameLoader.

Replace HistoryController::currentItem() with itemForNewChildFrame(), to clarify its sole remaining usage.

HistoryController is still owned by Page (and Page still manages deferring history navigations), but otherwise is referenced entirely by Source/web/. It can be moved out of core in a later patch.

BUG=none

Review URL: https://codereview.chromium.org/73123004

git-svn-id: svn://svn.chromium.org/blink/trunk@162495 bbb929c8-8fbe-4397-9dbb-9b2b20218538