Remove default args to several PageWidgetDelegate members.

No one passes the correct local root here yet; eventually, something....

BUG=411993

Review URL: https://codereview.chromium.org/603883002

git-svn-id: svn://svn.chromium.org/blink/trunk@182705 bbb929c8-8fbe-4397-9dbb-9b2b20218538