Blink GC plugin: Gracefully handle cyclic part objects.

tools/clang/blink_gc_plugin/tests/fields_require_tracing.txt

 fields_require_tracing.cpp:9:1: warning: [blink-gc] Class 'PartObject' has untraced fields that require tracing.
 void PartObject::trace(Visitor* visitor) {
 ^
-./fields_require_tracing.h:19:5: note: [blink-gc] Untraced field 'm_obj1' declared here:
+./fields_require_tracing.h:29:5: note: [blink-gc] Untraced field 'm_obj1' declared here:
     Member<HeapObject> m_obj1;
     ^
-./fields_require_tracing.h:21:5: note: [blink-gc] Untraced field 'm_obj3' declared here:
+./fields_require_tracing.h:31:5: note: [blink-gc] Untraced field 'm_obj3' declared here:
     Member<HeapObject> m_obj3;
     ^
-fields_require_tracing.cpp:15:1: warning: [blink-gc] Class 'HeapObject' has untraced fields that require tracing.
+fields_require_tracing.cpp:16:1: warning: [blink-gc] Class 'PartBObject' has untraced fields that require tracing.
+void PartBObject::trace(Visitor* visitor) {
+^
+./fields_require_tracing.h:20:5: note: [blink-gc] Untraced field 'm_set' declared here:
+    HeapHashSet<PartBObject> m_set;
+    ^
+fields_require_tracing.cpp:21:1: warning: [blink-gc] Class 'HeapObject' has untraced fields that require tracing.
 void HeapObject::trace(Visitor* visitor) {
 ^
-./fields_require_tracing.h:28:5: note: [blink-gc] Untraced field 'm_part' declared here:
+./fields_require_tracing.h:40:5: note: [blink-gc] Untraced field 'm_part' declared here:
     PartObject m_part;
     ^
-2 warnings generated.
+3 warnings generated.