Revert "DevTools: load panel modules lazily."

This reverts commit f43e2fe7b25446d0798752d9de12780b4a5adaf2.

Revert "DevTools: migrate extensible tabbed pane to promise-based instantiation."

This reverts commit dd1364de48e08ef526a5b02f2afe89e38debeab4.

Multiple browser_test failures following "load panel modules lazily"; the other
CL touched the same files afterwards, so reverting it as well.

TBR=pfeldman@chromium.org,hayato@chromium.org,aandrey@chromium.org
BUG=420087

Review URL: https://codereview.chromium.org/627733002

git-svn-id: svn://svn.chromium.org/blink/trunk@183202 bbb929c8-8fbe-4397-9dbb-9b2b20218538