Add a FrameLoader::m_currentItem and use it within FrameLoader.

This will eventually be the only history-related object Source/core needs to
know about. It will be exposed on FrameLoader in a later patch.

BUG=321280

Review URL: https://codereview.chromium.org/72863006

git-svn-id: svn://svn.chromium.org/blink/trunk@162317 bbb929c8-8fbe-4397-9dbb-9b2b20218538