blink::shutdown() should not touch a message loop

We're planning to destruct a message loop of the main thread
before Chromium calls blink::shutdown() (See https://codereview.chromium.org/583043005/).
Thus blink::shutdown() is not allowed to touch the message loop.
This CL removes code in blink::shutdown() that touches the message loop.

BUG=411026

Review URL: https://codereview.chromium.org/598903002

git-svn-id: svn://svn.chromium.org/blink/trunk@182655 bbb929c8-8fbe-4397-9dbb-9b2b20218538