BlinkGCPlugin: Handle omitted trace() in intermediary classes.

tools/clang/blink_gc_plugin/tests/traceimpl_omitted_trace.cpp

Index: tools/clang/blink_gc_plugin/tests/traceimpl_omitted_trace.cpp
diff --git a/content/common/presentation/presentation_session.mojom b/tools/clang/blink_gc_plugin/tests/traceimpl_omitted_trace.cpp
similarity index 72%
copy from content/common/presentation/presentation_session.mojom
copy to tools/clang/blink_gc_plugin/tests/traceimpl_omitted_trace.cpp
index 90f182135304df4f24061ceec31987d031345d74..b6dc2dffe589999c1be755a4ecef4f7dfd55e6ff 100644
--- a/content/common/presentation/presentation_session.mojom
+++ b/tools/clang/blink_gc_plugin/tests/traceimpl_omitted_trace.cpp
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-module presentation;
+#include "traceimpl_omitted_trace.h"
 
-interface PresentationSession {
-};
+// Nothing to define here.