As a first step, scheduling is extracted from ActiveDirectoryPolicyManager into a separate class.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dd04f963fac90b8d3502d8e96340b07be463b760

commit dd04f963fac90b8d3502d8e96340b07be463b760
Author: tnagel <tnagel@chromium.org>
Date: Wed Jun 21 17:46:25 2017

Extract AD policy scheduler into separate class

The new code should be easier to test, easier to extend and easier to
re-use.

BUG=734776
TEST=added unittests for scheduler

Review-Url: https://codereview.chromium.org/2942373002
Cr-Commit-Position: refs/heads/master@{#481237}

[modify] https://crrev.com/dd04f963fac90b8d3502d8e96340b07be463b760/chrome/browser/chromeos/policy/active_directory_policy_manager.cc
[modify] https://crrev.com/dd04f963fac90b8d3502d8e96340b07be463b760/chrome/browser/chromeos/policy/active_directory_policy_manager.h
[modify] https://crrev.com/dd04f963fac90b8d3502d8e96340b07be463b760/components/policy/core/common/BUILD.gn
[add] https://crrev.com/dd04f963fac90b8d3502d8e96340b07be463b760/components/policy/core/common/policy_scheduler.cc
[add] https://crrev.com/dd04f963fac90b8d3502d8e96340b07be463b760/components/policy/core/common/policy_scheduler.h
[add] https://crrev.com/dd04f963fac90b8d3502d8e96340b07be463b760/components/policy/core/common/policy_scheduler_unittest.cc

Next step is to merge CloudPolicyRefreshScheduler and PolicyScheduler.

For example, PolicyScheduler does not account for time the device has slept and thus would benefit from that piece of CloudPolicyRefreshScheduler functionality.