TracedValue.

Add a class for writing JSON-like structured data traces, composed from
primitive types like integers and strings as well as complex types like
dicts and arrays.

R=primiano@google.com,eseckler@google.com,skyostil@google.com

Change-Id: I85b011fb828f60ff570185d51ebf47b7dc074b7a
diff --git a/gn/perfetto_unittests.gni b/gn/perfetto_unittests.gni
index 44721ce..1a2b482 100644
--- a/gn/perfetto_unittests.gni
+++ b/gn/perfetto_unittests.gni
@@ -20,6 +20,7 @@
   "src/base:unittests",
   "src/protozero:unittests",
   "src/tracing/core:unittests",
+  "src/tracing:unittests",
   "src/profiling:unittests",
   "src/profiling/symbolizer:unittests",
 ]