[DevTools] Do not access WebViewImpl and WebDevToolsAgentClient when closing.

BUG=312393, 344623
R=dgozman@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/172463002

git-svn-id: svn://svn.chromium.org/blink/trunk@167585 bbb929c8-8fbe-4397-9dbb-9b2b20218538