This patch makes ContentIteratorBase, PostContentIterator, PreContentIterator
and ContentSubtreeIterator classes non-refcountable because most users can
create their instances in stack and such users may be in a hot path. So,
we can save a lot of cost of instantiation.
Unfortunately, only ScriptableContentIterator creates one of the concrete
classes and needs to destroy it properly. Therefore, its
EnsureContentIterator(), destructor, traverse and unlink code becomes messy.
However, ScriptableContentIterator was designed for automated tests and we
need to maintain it not so many times. Therefore, improvement of other
users must be worthwhiler than this demerit.
Differential Revision: https://phabricator.services.mozilla.com/D15928
--HG--
extra : moz-landing-system : lando
f421d7b889