This is a bit unfortunate, but otherwise we would have to include
objects.h before message.h, since for the initialization of a Handle<T>,
the compiler checks that Object* can be assigned to T*. So it would
need to know about the inheritance for initializing Handle<Script> and
Handle<JSFunction>.

R=mstarzinger@chromium.org, titzer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1872373002

Cr-Commit-Position: refs/heads/master@{#35626}