Fix header includes for brillo/supprocess

Chrome OS moved these files into a new directory a while
ago. Unfortunately in the last CrOS to AOSP merge, these were reverted
instead of being guarded by __CHROMEOS__. This CL does that.

Bug: 163153182
Test: FEATURES=test emerge-reef update_engine

Change-Id: If56cf95b9cecbb2f9dee74e41636bd90f9ace60b
diff --git a/common/subprocess.h b/common/subprocess.h
index 432d4cb..179a5c5 100644
--- a/common/subprocess.h
+++ b/common/subprocess.h
@@ -30,8 +30,13 @@
 #include <base/macros.h>
 #include <brillo/asynchronous_signal_handler_interface.h>
 #include <brillo/message_loops/message_loop.h>
+#ifdef __CHROMEOS__
+#include <brillo/process/process.h>
+#include <brillo/process/process_reaper.h>
+#else
 #include <brillo/process.h>
 #include <brillo/process_reaper.h>
+#endif  // __CHROMEOS__
 #include <gtest/gtest_prod.h>  // for FRIEND_TEST
 
 // The Subprocess class is a singleton. It's used to spawn off a subprocess