IDL: Clean-up "should use Nullable<>" logic a little

Some duplication had sneaked in as part of the IDL dictionary work, with
IdlTypeBase.impl_should_use_nullable_container meaning almost the same
thing as IdlTypeBase.is_explicit_nullable, except the latter also implies
that the type is nullable.

Common up the actual logic in the new IdlTypeBase.cpp_type_has_null_value
property, and use it for both impl_should_use_nullable_container and
is_{implicit,explicit}_nullable.

Review URL: https://codereview.chromium.org/472803002

git-svn-id: svn://svn.chromium.org/blink/trunk@180287 bbb929c8-8fbe-4397-9dbb-9b2b20218538