DevTools: Do not automatically show first panel when loading

This fixes the bug when last active panel was not revealed on inspector start.

R=pfeldman

Review URL: https://codereview.chromium.org/54693002

git-svn-id: svn://svn.chromium.org/blink/trunk@161072 bbb929c8-8fbe-4397-9dbb-9b2b20218538