Add the ability for objects which derive from RefCountedThreadSafe to specify a destructor trait. This allows browser objects to specify which thread they're terminated on. The benefit is we avoid the need to do manual ref counting when an object posts tasks to itself on different threads, if an object must be destructed on a specific thread.
This patch adds initial support and only shows one example with ResourceMessageFilter. I will do the rest in a follow-up patch to keep things small.
TEST=added unit tests