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

BUILD

diff --git a/BUILD b/BUILD
index 2579427..22940c9 100644
--- a/BUILD
+++ b/BUILD
@@ -524,6 +524,7 @@
         "include/perfetto/tracing/locked_handle.h",
         "include/perfetto/tracing/platform.h",
         "include/perfetto/tracing/trace_writer_base.h",
+        "include/perfetto/tracing/traced_value.h",
         "include/perfetto/tracing/tracing.h",
         "include/perfetto/tracing/tracing_backend.h",
         "include/perfetto/tracing/track.h",
@@ -1570,6 +1571,7 @@
         "src/tracing/internal/tracing_muxer_impl.h",
         "src/tracing/internal/track_event_internal.cc",
         "src/tracing/platform.cc",
+        "src/tracing/traced_value.cc",
         "src/tracing/tracing.cc",
         "src/tracing/track.cc",
         "src/tracing/track_event_category_registry.cc",