Do not use templates for ScriptPromiseResolver::ToV8 implementation

The usage of templates for reusing the internal implementation of ToV8
is not making the code simpler. So, inline the implementations.

This also fixes a GCC build issue because it was implemented with
template specializations declared in the class scope, and that is
hitting the GCC bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85282

Bug: 819294
Change-Id: I51f5a9e0a6e80ac707b630f270179c29fd84b059
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5223626
Commit-Queue: José Dapena Paz <jdapena@igalia.com>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1257835}