Previously NetworkMonitor required that a callback is passed to the
Create() method. It was hard to use and inconsistent with other APIs.
Added new UpdateNetworkList() method which accepts a
CompletionCallback to be called when network list changes.
BUG=281781
Review URL: https://chromiumcodereview.appspot.com/23453025
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221582 0039d316-1c4b-4281-b951-d872f2087c98