SkPDF: Refactor SkPDFObject heiararchy.

Flatten and use a tagged union type

        Δmemory ~= -2.4% ± 0.5%
        Δtime   ~= -1.2% ± 0.2%

BUG=skia:3585

Review URL: https://codereview.chromium.org/1069103003
7 files changed