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/testrunner.cc b/testrunner.cc
index 35295a9..354394b 100644
--- a/testrunner.cc
+++ b/testrunner.cc
@@ -20,7 +20,7 @@
 
 #include <base/at_exit.h>
 #include <base/command_line.h>
-#include <chromeos/test_helpers.h>
+#include <brillo/test_helpers.h>
 #include <gtest/gtest.h>
 
 #include "update_engine/terminator.h"