Loosen FrameLoaderStateMachine transition requirements

Instead of requiring transitions of 1 state, allow skipping states as long
as we never go backwards.

This means we don't have to do several transitions one right after the other
when we go out of the normal order (e.g., an explicit open in an initial
empty document).

BUG=264848

Review URL: https://chromiumcodereview.appspot.com/22912002

git-svn-id: svn://svn.chromium.org/blink/trunk@155963 bbb929c8-8fbe-4397-9dbb-9b2b20218538