Add TraceProviderRegistry and implementation in tracing service/application.

mojo/services/tracing/interfaces/BUILD.gn

Index: mojo/services/tracing/interfaces/BUILD.gn
diff --git a/mojo/services/tracing/interfaces/BUILD.gn b/mojo/services/tracing/interfaces/BUILD.gn
index 8d7ab59bad9508af935b48fb9fd4650127c503ee..18af9daa282c90dcb63009da412cead02667e004 100644
--- a/mojo/services/tracing/interfaces/BUILD.gn
+++ b/mojo/services/tracing/interfaces/BUILD.gn
@@ -7,6 +7,9 @@ import("$mojo_sdk_root/mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
   sources = [
+    "trace_provider_registry.mojom",
     "tracing.mojom",
   ]
+
+  import_dirs = [ get_path_info("../../", "abspath") ]
 }