DevTools: follow-up patch after r183608

We have to clear sessionId not only at the end but also after disconnecting frontend.
Othervise it will be not null for the next devtools session and might be republished at navigation and break tracing session for the other tab.

BUG=

Review URL: https://codereview.chromium.org/657383002

git-svn-id: svn://svn.chromium.org/blink/trunk@183867 bbb929c8-8fbe-4397-9dbb-9b2b20218538