DevTools: use MainFrameNavigated, not DocumentUpdated to re-enable Tracing agent to avoid races with DOMModel loading document.

BUG=

Review URL: https://codereview.chromium.org/618803002

git-svn-id: svn://svn.chromium.org/blink/trunk@182969 bbb929c8-8fbe-4397-9dbb-9b2b20218538