[DevTools] Shutdown inspector before detaching frame.

Inspector extensively uses frame and view during shutdown, so
it should be destroyed first. Added assert to check this.

BUG=409726

Review URL: https://codereview.chromium.org/538483002

git-svn-id: svn://svn.chromium.org/blink/trunk@181315 bbb929c8-8fbe-4397-9dbb-9b2b20218538