Before this change a malformed value of RemoteAccessHostUdpPortRange policy
(i.e. "123456-blah") was ignored and a default value was used.  Similarily for
the 3 third-party-auth-config policies (RemoteAccessHostTokenUrl,
...TokenValidationUrl and ...TokenValidationCertificateIssuer) we were falling
back to a secure default, but not reporting a policy error.  After this change
such malformed values will trigger an OnPolicyError callback.

Notes:
- Guaranteeing that PolicyWatcher always returns valid policy values, removes
  the need for a "rejecting" Me2MeHostAuthenticatorFactory.
- Moving PortRange and ThirdPartyAuthConfig to separate compilation units
  helps readability elsewhere + encourages better unit tests coverage.
- Initially I tried to wrap all policies in a new ChromotingPolicies class,
  but eventually went back to prevalidating and passing base::DictionaryValue.
  - Arguments for using ChromotingPolicies:
    - Helps avoid overtesting in policy_watcher_unittests.cc (i.e. helps focus
      the tests on a single policy value).
  - Arguments for using base::DictionaryValue:
    - Minimizes changes.
    - Keeps things simple (as opposed to having to introduce a custom
      equivalent of optional<T> [nothing similar present in Chromium AFAICT]).
  - Neutral:
    - Strong-typing of ChromotingPolicies didn't help readability as much as I
      expected and hoped for.

BUG=427513
TEST=remoting_unittests

Review URL: https://codereview.chromium.org/966433002

Cr-Commit-Position: refs/heads/master@{#318910}