Fix incorrect iframe loading during back/forward navigation.

This is a partial revert of blink r156330. It adds back in a special case
for a new JS-initiated navigation before the first real load in an iframe, where
we cancel the existing provisional load.

BUG=374174,348447
TEST=http/tests/navigation/back-to-dynamic-iframe.html

Review URL: https://codereview.chromium.org/296533004

git-svn-id: svn://svn.chromium.org/blink/trunk@174324 bbb929c8-8fbe-4397-9dbb-9b2b20218538