Reland [skjson] Implementation/API tweaks

  *  move most common accessor methods to the header, for inlining
  *  drop the lazy type checking semantics in favor of explicit guarded/unguarded
     conversions
  *  revisit the public class hierarchy to better constrain type-bound APIs
  *  expose public type factories and add tests
  *  drop the empty-vector optimization -- allocating an external size_t in these
     uncommon cases is better than paying for a conditional on every access.

TBR=
Change-Id: Ic609bb74f12cad1756865a2489ad56c03ecc5494
Reviewed-on: https://skia-review.googlesource.com/134845
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed