DevTools: NetworkPanel: add "Connection Id" column.

Developers can use this column to see what resources share connection.

BUG=

Review URL: https://codereview.chromium.org/513703002

git-svn-id: svn://svn.chromium.org/blink/trunk@181461 bbb929c8-8fbe-4397-9dbb-9b2b20218538