[skottie] Json cleanup pass

Assorted Json tweaks:

  - more defensive internal object access
  - drop unneeded isObject checks
  - drop unneeded namespace
  - restrict the iterator to arrays

TBR=
Change-Id: I02f1c5d84c429cf5206bc2a0a7843097b92bac94
Reviewed-on: https://skia-review.googlesource.com/126930
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed