Adds type members and accessors to the NetworkStateNotifier.  Also allows for registration of thread-safe observers for said type information so that both workers and windows can receive notification.

This is part of adding NetInfo v3 to Chrome, and this code is used by https://codereview.chromium.org/291203002.

Design doc: https://docs.google.com/a/chromium.org/document/d/1LTk9uVMGi4kurzcF5ellsAJReTF31fFJMHrQwSVtBjc/ 

BUG=368358

Review URL: https://codereview.chromium.org/289333003

git-svn-id: svn://svn.chromium.org/blink/trunk@174640 bbb929c8-8fbe-4397-9dbb-9b2b20218538