DevTools: register panels prior to opening views in drawer.

Working on a test for it since it is non-trivial.

TBR=vsevik
BUG=335790

Review URL: https://codereview.chromium.org/133273034

git-svn-id: svn://svn.chromium.org/blink/trunk@165364 bbb929c8-8fbe-4397-9dbb-9b2b20218538