Don't print out **nav target** when dumpBackForwardList() is called.

**nav target** is printed when HistoryItem::isTargetItem is
true and is the only use of that flag. It will be deleted in
a later patch.

Also, change some tests in http/tests/navigation to use
dumpAsText(), since it made generating new baselines for
this patch easier and there's no reason they need to dump a
render tree.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/24252015

git-svn-id: svn://svn.chromium.org/blink/trunk@158214 bbb929c8-8fbe-4397-9dbb-9b2b20218538