Revert "Revert "[DevTools] HAR: add "connection" field.""

This reverts r156386, apparently r156364 is not to blame for broken
inspector-protocol/runtime-getProperties.html.

TBR=eustas@chromium.org
BUG=276262

Review URL: https://codereview.chromium.org/23273003

git-svn-id: svn://svn.chromium.org/blink/trunk@156388 bbb929c8-8fbe-4397-9dbb-9b2b20218538