GitLab

Removal of FakePolicyWatcher means that PolicyServiceWatcher is now the only
class derived from PolicyWatcher.  A next check-in should merge PolicyWatcher
and PolicyServiceWatcher into a single class.

Testing via FakeAsyncPolicyLoader rather than FakePolicyWatcher means that
tests cover more of PolicyServiceWatcher code.

I chose to introduce FakeAsyncPolicyLoader, rather than going via an already
existing MockConfigurationPolicyProvier, because 1) this way I can cover more
of PolicyServiceWatcher code and 2) this simplifies the test code (by a) keeping
PolicySchema manipulation within the product code and b) not requiring mocking
of IsInitializationComplete and c) being able to directly call into unmodified
CreateFromPolicyLoader method in the product code).

BUG=368321

Review URL: https://codereview.chromium.org/858303003

Cr-Commit-Position: refs/heads/master@{#312994}