Make createWindow (mostly) work with OOPIF

      If a RemoteFrame is targeted by window.open(), this change will allow us to
correctly plumb the navigation out of blink.

We no longer assume that ChromeClient::createWindow will return a Page with a
local main frame. However, there are a several places where we wouldn't have
the plumbing to set the correct state even if a Page with a remote main frame
was ever returned.

This is a fix+reland of https://codereview.chromium.org/1109213002,
with a lot less cruft.

BUG=463742

Review URL: https://codereview.chromium.org/1126133005

git-svn-id: svn://svn.chromium.org/blink/trunk@195424 bbb929c8-8fbe-4397-9dbb-9b2b20218538