Let Dart code running in Sky add events to the trace timeline

sky/engine/core/frame/Tracing.idl

Index: sky/engine/core/frame/Tracing.idl
diff --git a/tools/clang/blink_gc_plugin/tests/traceimpl_derived_from_templated_base.cpp b/sky/engine/core/frame/Tracing.idl
similarity index 67%
copy from tools/clang/blink_gc_plugin/tests/traceimpl_derived_from_templated_base.cpp
copy to sky/engine/core/frame/Tracing.idl
index 9636fca727a446ced9c81bc31f860010a909d4aa..7f3e407c06739bdd93908058aeb2ca44b2230680 100644
--- a/tools/clang/blink_gc_plugin/tests/traceimpl_derived_from_templated_base.cpp
+++ b/sky/engine/core/frame/Tracing.idl
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "traceimpl_derived_from_templated_base.h"
-
-// Nothing to define.
+interface Tracing {
+  void begin(DOMString name);
+  void end(DOMString name);
+};