Add chrome.sockets.udp.setPaused method.

* Add test
* Make a couple of callback optional for consistency across APIs.

BUG=173241
BUG=165273

Review URL: https://codereview.chromium.org/109143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239524 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239524