Also use DevicePolicy::GetAuP2PEnabled() to determine if p2p is enabled

With this change update_engine will use p2p either if the user has
manually enabled it (through the crosh flag) OR if the enterprise has
enabled it in Enterprise Policy.

BUG=chromium:260442
TEST=New unit tests + unit tests pass.

Change-Id: I54cd92c481bd2fd7c90232d7137ce2b37fa2ce61
Reviewed-on: https://chromium-review.googlesource.com/168950
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/fake_p2p_manager.h b/fake_p2p_manager.h
index 87788bf..aebc238 100644
--- a/fake_p2p_manager.h
+++ b/fake_p2p_manager.h
@@ -22,6 +22,8 @@
   virtual ~FakeP2PManager() {}
 
   // P2PManager overrides.
+  virtual void SetDevicePolicy(const policy::DevicePolicy* device_policy) {}
+
   virtual bool IsP2PEnabled() {
     return is_p2p_enabled_;
   }