blink_gc_plugin: retire overloaded traceImpl detection and handling.

tools/clang/blink_gc_plugin/tests/traceimpl_dependent_scope.txt

Index: tools/clang/blink_gc_plugin/tests/traceimpl_dependent_scope.txt
diff --git a/tools/clang/blink_gc_plugin/tests/traceimpl_dependent_scope.txt b/tools/clang/blink_gc_plugin/tests/traceimpl_dependent_scope.txt
index b4779a09d9a277587862ad87a4babefb224eb4f2..66c1d267ae0fa94e200a8d207ad79a35a558e7d0 100644
--- a/tools/clang/blink_gc_plugin/tests/traceimpl_dependent_scope.txt
+++ b/tools/clang/blink_gc_plugin/tests/traceimpl_dependent_scope.txt
@@ -1,5 +1,5 @@
 In file included from traceimpl_dependent_scope.cpp:5:
-./traceimpl_dependent_scope.h:55:3: warning: [blink-gc] Base class 'Base<int>' of derived class 'DerivedMissingTrace<int>' requires tracing.
-  void TraceImpl(VisitorDispatcher visitor) {
+./traceimpl_dependent_scope.h:32:3: warning: [blink-gc] Base class 'Base<int>' of derived class 'DerivedMissingTrace<int>' requires tracing.
+  void Trace(Visitor* visitor) override {
   ^
 1 warning generated.