Fold SkJSON into Skia/utils

It's a tiny, core-ish component -- might as well treat as such to
simplify dependencies.

Change-Id: I6f31ce2d151f9a629d88bfc7f15d64891d5150c0
Reviewed-on: https://skia-review.googlesource.com/135780
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/gn/tests.gni b/gn/tests.gni
index 5f028e8..32c3337 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -123,6 +123,7 @@
   "$_tests/InterpolatorTest.cpp",
   "$_tests/InvalidIndexedPngTest.cpp",
   "$_tests/IsClosedSingleContourTest.cpp",
+  "$_tests/JSONTest.cpp",
   "$_tests/LayerDrawLooperTest.cpp",
   "$_tests/LazyProxyTest.cpp",
   "$_tests/LListTest.cpp",