DevTools: fix console getting broken if reloaded with active filter

This patch moves target observing to the end of ConsoleView constructor
so that all the objects are created prior the targetAdded callbacks.

Review URL: https://codereview.chromium.org/237533006

git-svn-id: svn://svn.chromium.org/blink/trunk@171679 bbb929c8-8fbe-4397-9dbb-9b2b20218538