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/proxy_resolver.cc b/proxy_resolver.cc
index 7448b8d..abd6f76 100644
--- a/proxy_resolver.cc
+++ b/proxy_resolver.cc
@@ -19,7 +19,7 @@
 #include <base/bind.h>
 #include <base/location.h>
 
-using chromeos::MessageLoop;
+using brillo::MessageLoop;
 using std::deque;
 using std::string;