Revert r167585 "[DevTools] Do not access WebViewImpl and WebDevToolsAgentClient when closing." Debug bots are unhappy.

TBR=haraken

Review URL: https://codereview.chromium.org/173893010

git-svn-id: svn://svn.chromium.org/blink/trunk@167593 bbb929c8-8fbe-4397-9dbb-9b2b20218538