DevTools: [SSP] implement "add rule to end" of arbitrary style sheet.

The patch adds long click behavior to the "add new rule" button, which
opens a list of user stylesheets where you can add a new rule.

BUG=402461

Review URL: https://codereview.chromium.org/529553002

git-svn-id: svn://svn.chromium.org/blink/trunk@181552 bbb929c8-8fbe-4397-9dbb-9b2b20218538