DevTools: Fix the way extension sidebar panes are added to elements panel and imlement the same for the sources panel.

R=caseq
BUG=295888

Review URL: https://chromiumcodereview.appspot.com/23537066

git-svn-id: svn://svn.chromium.org/blink/trunk@158287 bbb929c8-8fbe-4397-9dbb-9b2b20218538