In file included from register_weak_members_template.cpp:5:
./register_weak_members_template.h:19:3: warning: [blink-gc] Class 'HasUntracedWeakMembers' has untraced fields that require tracing.
  void Trace(Visitor* visitor) {
  ^
./register_weak_members_template.h:28:3: note: [blink-gc] Untraced field 'x_' declared here:
  WeakMember<X> x_;
  ^
1 warning generated.
