SkPDF: Add sk_sp setters; .release() becomes std::move()

SkPDF: Add sk_sp setters; .release() becomes std::move()

Note to reviewers:  Start with changes to SkPDFTypes.h

Many places that had a bare pointer owning a reference are refactored to
use a sk_sp.

There remain several places where a non-owning pointer `T*` should be
replaced with `const sk_sp<T>&` to eliminate the common pattern
`sk_sp<T>(SkRef(x))`.

Review URL: https://codereview.chromium.org/1775043002