[skottie] Switch to SkJSON

Convert to SkJSON APIs and remove RapidJSON dependency.

TBR=
Change-Id: I5b4fd78821a443326e3a5b370748d840b80ef279
Reviewed-on: https://skia-review.googlesource.com/134612
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/DEPS b/DEPS
index 4cac345..a0925bc 100644
--- a/DEPS
+++ b/DEPS
@@ -12,7 +12,7 @@
   "third_party/externals/harfbuzz"        : "https://skia.googlesource.com/third_party/harfbuzz.git@1.4.2",
   "third_party/externals/icu"             : "https://chromium.googlesource.com/chromium/deps/icu.git@ec9c1133693148470ffe2e5e53576998e3650c1d",
   "third_party/externals/imgui"           : "https://skia.googlesource.com/external/github.com/ocornut/imgui.git@6384eee34f08cb7eab8d835043e1738e4adcdf75",
-  # TODO: remove jsoncpp after migrating clients to rapidjson
+  # TODO: remove jsoncpp after migrating clients to SkJSON
   "third_party/externals/jsoncpp"         : "https://chromium.googlesource.com/external/github.com/open-source-parsers/jsoncpp.git@1.0.0",
   "third_party/externals/libjpeg-turbo"   : "https://skia.googlesource.com/external/github.com/libjpeg-turbo/libjpeg-turbo.git@1.5.3",
   "third_party/externals/libpng"          : "https://skia.googlesource.com/third_party/libpng.git@v1.6.33",
@@ -21,7 +21,6 @@
   "third_party/externals/microhttpd"      : "https://android.googlesource.com/platform/external/libmicrohttpd@748945ec6f1c67b7efc934ab0808e1d32f2fb98d",
   "third_party/externals/opengl-registry" : "https://skia.googlesource.com/external/github.com/KhronosGroup/OpenGL-Registry@14b80ebeab022b2c78f84a573f01028c96075553",
   "third_party/externals/piex"            : "https://android.googlesource.com/platform/external/piex.git@bb217acdca1cc0c16b704669dd6f91a1b509c406",
-  "third_party/externals/rapidjson"       : "https://skia.googlesource.com/external/github.com/Tencent/rapidjson.git@af223d44f4e8d3772cb1ac0ce8bc2a132b51717f",
   "third_party/externals/sdl"             : "https://skia.googlesource.com/third_party/sdl@5d7cfcca344034aff9327f77fc181ae3754e7a90",
   "third_party/externals/sfntly"          : "https://chromium.googlesource.com/external/github.com/googlei18n/sfntly.git@b18b09b6114b9b7fe6fc2f96d8b15e8a72f66916",
   "third_party/externals/spirv-headers"   : "https://skia.googlesource.com/external/github.com/KhronosGroup/SPIRV-Headers.git@661ad91124e6af2272afd00f804d8aa276e17107",