DevTools: Improve pretty print UX

 - Enables pretty-printing on reload
  - Supports pretty-rpinting HTML document without inline scripts

  BUG=378261, 389036

Review URL: https://codereview.chromium.org/428943002

git-svn-id: svn://svn.chromium.org/blink/trunk@179222 bbb929c8-8fbe-4397-9dbb-9b2b20218538