Add Blink-side support for StartClosingHandshake

Add the Blink-side support for the OnClosingHandshake message from the
browser. Changes to content/ are still required before this does anything.

BUG=340904

Review URL: https://codereview.chromium.org/153563005

git-svn-id: svn://svn.chromium.org/blink/trunk@166805 bbb929c8-8fbe-4397-9dbb-9b2b20218538