Merge FrameLoader history loading functions

Most of loadSameDocumentItem is unnecessary and loadDifferentDocumentItem is
reasonably short. They're both only called from loadHistoryItem, so merge
them in there.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23033015

git-svn-id: svn://svn.chromium.org/blink/trunk@156483 bbb929c8-8fbe-4397-9dbb-9b2b20218538