Address FIXME in CreateWindow: use FocusController to change focus

The main difference is that going via the ChromeClient will also raise
the window. This is, however, undesirable, as a page can open its own
frames without limitations, and therefore can use this to create
popunders

BUG=343064
R=mkwst@chromium.org

Review URL: https://codereview.chromium.org/158953009

git-svn-id: svn://svn.chromium.org/blink/trunk@167000 bbb929c8-8fbe-4397-9dbb-9b2b20218538