Avoid leaking memory under certain conditions

Avoid leaking memory under certain conditions

When document is loaded pages that are already rendered are not
immediately freed, futhermore the eviction will not happen given that
old pages occupy the same spot and are not considered as eviction
targets, generation count is not checked and even if it was eviting
old page with the new underlying document can be unsafe (depends on
how the underlying rendering engine is implemented), so the safest way
to deal with it is to just flush them en masse.