blink_gc_plugin: retire overloaded traceImpl detection and handling.

tools/clang/blink_gc_plugin/tests/traceimpl_overloaded_error.txt

Index: tools/clang/blink_gc_plugin/tests/traceimpl_overloaded_error.txt
diff --git a/tools/clang/blink_gc_plugin/tests/traceimpl_overloaded_error.txt b/tools/clang/blink_gc_plugin/tests/traceimpl_overloaded_error.txt
index b603a0861ffb69306ee0ee3aa09c1eead487e343..eccfb037d0f0d424777b380d1a492fc70c68ded1 100644
--- a/tools/clang/blink_gc_plugin/tests/traceimpl_overloaded_error.txt
+++ b/tools/clang/blink_gc_plugin/tests/traceimpl_overloaded_error.txt
@@ -1,28 +1,28 @@
 In file included from traceimpl_overloaded_error.cpp:5:
-./traceimpl_overloaded_error.h:27:3: warning: [blink-gc] Class 'InlinedBase' has untraced fields that require tracing.
-  void TraceImpl(VisitorDispatcher visitor) {
+./traceimpl_overloaded_error.h:19:3: warning: [blink-gc] Class 'InlinedBase' has untraced fields that require tracing.
+  virtual void Trace(Visitor* visitor) {
   ^
-./traceimpl_overloaded_error.h:31:3: note: [blink-gc] Untraced field 'x_base_' declared here:
+./traceimpl_overloaded_error.h:24:3: note: [blink-gc] Untraced field 'x_base_' declared here:
   Member<X> x_base_;
   ^
-./traceimpl_overloaded_error.h:43:3: warning: [blink-gc] Base class 'InlinedBase' of derived class 'InlinedDerived' requires tracing.
-  void TraceImpl(VisitorDispatcher visitor) {
+./traceimpl_overloaded_error.h:29:3: warning: [blink-gc] Base class 'InlinedBase' of derived class 'InlinedDerived' requires tracing.
+  void Trace(Visitor* visitor) override {
   ^
-./traceimpl_overloaded_error.h:43:3: warning: [blink-gc] Class 'InlinedDerived' has untraced fields that require tracing.
-./traceimpl_overloaded_error.h:47:3: note: [blink-gc] Untraced field 'x_derived_' declared here:
+./traceimpl_overloaded_error.h:29:3: warning: [blink-gc] Class 'InlinedDerived' has untraced fields that require tracing.
+./traceimpl_overloaded_error.h:34:3: note: [blink-gc] Untraced field 'x_derived_' declared here:
   Member<X> x_derived_;
   ^
-traceimpl_overloaded_error.cpp:18:1: warning: [blink-gc] Class 'ExternBase' has untraced fields that require tracing.
-inline void ExternBase::TraceImpl(VisitorDispatcher visitor) {
+traceimpl_overloaded_error.cpp:9:1: warning: [blink-gc] Class 'ExternBase' has untraced fields that require tracing.
+void ExternBase::Trace(Visitor* visitor) {
 ^
-./traceimpl_overloaded_error.h:59:3: note: [blink-gc] Untraced field 'x_base_' declared here:
+./traceimpl_overloaded_error.h:42:3: note: [blink-gc] Untraced field 'x_base_' declared here:
   Member<X> x_base_;
   ^
-traceimpl_overloaded_error.cpp:31:1: warning: [blink-gc] Base class 'ExternBase' of derived class 'ExternDerived' requires tracing.
-inline void ExternDerived::TraceImpl(VisitorDispatcher visitor) {
+traceimpl_overloaded_error.cpp:13:1: warning: [blink-gc] Base class 'ExternBase' of derived class 'ExternDerived' requires tracing.
+void ExternDerived::Trace(Visitor* visitor) {
 ^
-traceimpl_overloaded_error.cpp:31:1: warning: [blink-gc] Class 'ExternDerived' has untraced fields that require tracing.
-./traceimpl_overloaded_error.h:71:3: note: [blink-gc] Untraced field 'x_derived_' declared here:
+traceimpl_overloaded_error.cpp:13:1: warning: [blink-gc] Class 'ExternDerived' has untraced fields that require tracing.
+./traceimpl_overloaded_error.h:50:3: note: [blink-gc] Untraced field 'x_derived_' declared here:
   Member<X> x_derived_;
   ^
 6 warnings generated.