Mojo: Make mojo::Handle::value() return a reference instead of a value.

mojo/public/cpp/system/handle.h

Index: mojo/public/cpp/system/handle.h
diff --git a/mojo/public/cpp/system/handle.h b/mojo/public/cpp/system/handle.h
index af81824603f6d00a7542b2173206105d551b93ee..7aa20f521787e161fd6da542a59245322c703a50 100644
--- a/mojo/public/cpp/system/handle.h
+++ b/mojo/public/cpp/system/handle.h
@@ -174,7 +174,7 @@ class Handle {
     return value_ != kInvalidHandleValue;
   }
 
-  MojoHandle value() const { return value_; }
+  const MojoHandle& value() const { return value_; }
   MojoHandle* mutable_value() { return &value_; }
   void set_value(MojoHandle value) { value_ = value; }