[DevTools] Add JSDocs / minor refactoring of RequestHeadersView.js

EOM

BUG=

Review URL: https://chromiumcodereview.appspot.com/24345004

git-svn-id: svn://svn.chromium.org/blink/trunk@158277 bbb929c8-8fbe-4397-9dbb-9b2b20218538