BlinkGCPlugin: Fix base class trace call detection in traceImpl().

tools/clang/blink_gc_plugin/tests/traceimpl_error.txt

Index: tools/clang/blink_gc_plugin/tests/traceimpl_error.txt
diff --git a/tools/clang/blink_gc_plugin/tests/traceimpl_error.txt b/tools/clang/blink_gc_plugin/tests/traceimpl_error.txt
index e2471beaeeb74b3f8e68e6ba38a454ec3aea555b..070b02961c0953d3fc2ba8674ec4e158fa1471a1 100644
--- a/tools/clang/blink_gc_plugin/tests/traceimpl_error.txt
+++ b/tools/clang/blink_gc_plugin/tests/traceimpl_error.txt
@@ -5,10 +5,16 @@ In file included from traceimpl_error.cpp:5:
 ./traceimpl_error.h:28:3: note: [blink-gc] Untraced field 'x_' declared here:
   Member<X> x_;
   ^
+./traceimpl_error.h:53:3: warning: [blink-gc] Base class 'Base' of derived class 'TraceImplInlineWithUntracedBase' requires tracing.
+  void traceImpl(VisitorDispatcher visitor) {
+  ^
 traceimpl_error.cpp:14:1: warning: [blink-gc] Class 'TraceImplExternWithUntracedMember' has untraced fields that require tracing.
 inline void TraceImplExternWithUntracedMember::traceImpl(
 ^
-./traceimpl_error.h:41:3: note: [blink-gc] Untraced field 'x_' declared here:
+./traceimpl_error.h:40:3: note: [blink-gc] Untraced field 'x_' declared here:
   Member<X> x_;
   ^
-2 warnings generated.
+traceimpl_error.cpp:24:1: warning: [blink-gc] Base class 'Base' of derived class 'TraceImplExternWithUntracedBase' requires tracing.
+inline void TraceImplExternWithUntracedBase::traceImpl(
+^
+4 warnings generated.