DevTools: reapply r167585: Do not access WebViewImpl and WebDevToolsAgentClient when closing.

TBR=yurys
BUG=312393, 344623i

Review URL: https://codereview.chromium.org/175313002

git-svn-id: svn://svn.chromium.org/blink/trunk@169115 bbb929c8-8fbe-4397-9dbb-9b2b20218538