update_engine: Rename "chromeos" -> "brillo" in include paths and namespaces

libchromeos is transitioning to libbrillo and chromeos namespaces
and include directory is changing to brillo.

Bug: 24872993
Change-Id: I770659a95be380a50fe3b2ba9f91d65818f40945
diff --git a/p2p_manager.cc b/p2p_manager.cc
index 3cba737..96aaaaf 100644
--- a/p2p_manager.cc
+++ b/p2p_manager.cc
@@ -58,7 +58,7 @@
 using base::StringPrintf;
 using base::Time;
 using base::TimeDelta;
-using chromeos::MessageLoop;
+using brillo::MessageLoop;
 using chromeos_update_manager::EvalStatus;
 using chromeos_update_manager::Policy;
 using chromeos_update_manager::UpdateManager;