Adding PrintTo of ObjectPath for gtest.

dbus/object_path.h

Index: dbus/object_path.h
diff --git a/dbus/object_path.h b/dbus/object_path.h
index 89c5287c54a6706850d8ecb0f6007ed1f3e29826..072c23daae6d1aaac34e382077c81664e88feaa1 100644
--- a/dbus/object_path.h
+++ b/dbus/object_path.h
@@ -5,6 +5,7 @@
 #ifndef DBUS_OBJECT_PATH_H_
 #define DBUS_OBJECT_PATH_H_
 
+#include <iosfwd>
 #include <string>
 
 #include "dbus/dbus_export.h"
@@ -46,6 +47,9 @@ class CHROME_DBUS_EXPORT ObjectPath {
   std::string value_;
 };
 
+// This is required by gtest to print a readable output on test failures.
+CHROME_DBUS_EXPORT void PrintTo(const ObjectPath& path, std::ostream* out);
+
 }  // namespace dbus
 
 #endif  // DBUS_OBJECT_PATH_H_