Revert "[skjson] Implementation/API tweaks"

This reverts commit 03b68421cafa58428edd8ab12e0d2235c7000c3f.

Reason for revert: Broke Debian9 builds

Original change's description:
> [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.
> 
> Change-Id: I24a7c75db3aa8b12c740c77ac7df4af4e3a1dff8
> Reviewed-on: https://skia-review.googlesource.com/134610
> Commit-Queue: Florin Malita <fmalita@chromium.org>
> Reviewed-by: Mike Klein <mtklein@google.com>

TBR=mtklein@google.com,fmalita@chromium.org

Change-Id: I2c681ef5c8d5fc15508e58b4b0f6ab9491b7d76f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/134880
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed