Add TraceProviderRegistry and implementation in tracing service/application.

mojo/services/tracing/interfaces/trace_provider_registry.mojom

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This file contains the top-level interface (provided by the tracing
+// application's ServiceProvider) used by things that want to register
+// themselves to participate in tracing.
+
+[DartPackage="mojo_services"]
+module tracing;
+
+import "tracing/interfaces/tracing.mojom";
+
+[ServiceName="tracing.TraceProviderRegistry"]
+interface TraceProviderRegistry {
+  RegisterTraceProvider(TraceProvider trace_provider);
+};