Stop using HistoryController::previousItem() in Internals.

This involves rewriting a few form state tests to be able to access the
relevant state on the current item instead.

BUG=321280

Review URL: https://codereview.chromium.org/59153015

git-svn-id: svn://svn.chromium.org/blink/trunk@162486 bbb929c8-8fbe-4397-9dbb-9b2b20218538