Add WindowFocusAllowedIndicator to the public API.

This is required to allow the chromium extension system to allow window.focus in certain situations.
BUG=325014

Review URL: https://codereview.chromium.org/108803004

git-svn-id: svn://svn.chromium.org/blink/trunk@164244 bbb929c8-8fbe-4397-9dbb-9b2b20218538