[skottie] Switch to RapidJSON

- pull latest RapidJSON under third_party/externals/rapidjson
  (note: and older RS version is already pulled as part of angle2,
  and it is also checked in G3)

- add a thin Json porting layer (SkottieJson) to isolate RS
  idiosyncrasies

- convert Skottie to use the new helpers

- parse the DOM in-place (based on local experiments this is the
  fastest method)

Ta-da: Skottie now parses JSON ~10x faster!


Change-Id: Ida9099638f88ed025fee83055c8cd8680ee27176
Reviewed-on: https://skia-review.googlesource.com/125744
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
11 files changed