[tracing] perfetto::Flow

Add perfetto::Flow and perfetto::TerminatingFlow helpers which can be
passed directly to TRACE_EVENT macros to populate the corresponding
proto fields.

R=eseckler@google.com

Change-Id: I7eaac2fdb0fe01872670f7b6754191adfe0fbd14
diff --git a/BUILD b/BUILD
index 264030b..57cbfbd 100644
--- a/BUILD
+++ b/BUILD
@@ -583,6 +583,7 @@
         "include/perfetto/tracing/tracing_policy.h",
         "include/perfetto/tracing/track.h",
         "include/perfetto/tracing/track_event.h",
+        "include/perfetto/tracing/track_event_args.h",
         "include/perfetto/tracing/track_event_category_registry.h",
         "include/perfetto/tracing/track_event_interned_data_index.h",
         "include/perfetto/tracing/track_event_legacy.h",