Initial iframe load shouldn't add session history item.

This CL is fixing the behavior of setReplacesCurrentHistoryItem for initial frame navigation. The Chromium side change is at  https://codereview.chromium.org/15294012/.

BUG=178380

Review URL: https://codereview.chromium.org/25269006

git-svn-id: svn://svn.chromium.org/blink/trunk@158723 bbb929c8-8fbe-4397-9dbb-9b2b20218538