Expose Frame::detach() to the Blink API layer.

This is needed when the embedder needs to trigger frame detach for frame
trees that have been mirrored into other processes. This patch also adds
a frameDetached() callback for remote frames so the embedder knows when
to close() the corresponding frame.

BUG=357747

Review URL: https://codereview.chromium.org/593893002

git-svn-id: svn://svn.chromium.org/blink/trunk@182625 bbb929c8-8fbe-4397-9dbb-9b2b20218538