Add TraceProviderRegistry and implementation in tracing service/application.

mojo/services/tracing/interfaces/tracing.mojom

Index: mojo/services/tracing/interfaces/tracing.mojom
diff --git a/mojo/services/tracing/interfaces/tracing.mojom b/mojo/services/tracing/interfaces/tracing.mojom
index b2300e03c621b4c1774b9e84e0840b5dc9e29be5..558bec3e3aaca98e387980cd8320ad3a81f39038 100644
--- a/mojo/services/tracing/interfaces/tracing.mojom
+++ b/mojo/services/tracing/interfaces/tracing.mojom
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// TODO(vtl): This file should be split into trace_provider.mojom (containing
+// TraceProvider and TraceRecorder) and trace_collector.mojom.
+
 [DartPackage="mojo_services"]
 module tracing;