static init(versionString) {
    goog.asserts.assert(
        shaka.Deprecate.enforcer_ == null,
        'Deprecate.init should only be called once.');

    shaka.Deprecate.enforcer_ = new shaka.deprecate.Enforcer(
        shaka.deprecate.Version.parse(versionString),
        shaka.Deprecate.onPending_,
        shaka.Deprecate.onExpired_);
  }
Initialize the system. This must happen before any calls to |enforce|. In
our code base, |shaka.Player| will be the only one to call this (it has the
version string).

If the |Deprecate| called |Player.version| to initialize itself, it would
mean that |Player| could not use |Deprecate| because it would create a
circular dependency. To work around this, we provide this method so that
|Player| can give us the version without us needing to know about |Player|.

This will initialize the system to:
 - print warning messages when the feature is scheduled to be removed in a
   later version
 - print errors and fail assertions when the feature should be removed now

@param {string} versionString