DevTools: Add public frame-related methods to Page domain.

Added frameAttached event and made existing frameNavigated and
frameDetached public. Also made Page.Frame type public as it is
used by frameNavigated.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/23565006

git-svn-id: svn://svn.chromium.org/blink/trunk@157222 bbb929c8-8fbe-4397-9dbb-9b2b20218538