Reapply "DevTools: migrate extensible tabbed pane to promise-based instantiation.", "DevTools: load panel modules lazily." reverted as r183202.

TBR=apavlov
BUG=420087

Review URL: https://codereview.chromium.org/630703002

git-svn-id: svn://svn.chromium.org/blink/trunk@183236 bbb929c8-8fbe-4397-9dbb-9b2b20218538