DevTools: Support inplace formatting for snippets.

R=pfeldman@chromium.org, pfeldman
BUG=347496

Review URL: https://codereview.chromium.org/176843007

git-svn-id: svn://svn.chromium.org/blink/trunk@168311 bbb929c8-8fbe-4397-9dbb-9b2b20218538