Introduce legacy JSON importer and "slices" vtable

First step to support Chromium legacy JSON traces.
The support is very minimal for the moment, but good enough
to unblock the UI work.

Bug: 112295162
Change-Id: I23b7ccbb9c9ebe706fbbdd56c1ac417c3f37df6d
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 682d085..92f4dee 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -138,3 +138,14 @@
 config("fuzzer_config") {
   ldflags = [ "-fsanitize=fuzzer" ]
 }
+
+# For now JsonCpp is supported only in standalone builds outside of Android or
+# Chromium.
+group("jsoncpp_deps") {
+  if (build_standalone) {
+    public_configs = [ "//buildtools:jsoncpp_config" ]
+    public_deps = [
+      "//buildtools:jsoncpp",
+    ]
+  }
+}