Disallow heap objects containing unsafe on-heap iterators.

tools/clang/blink_gc_plugin/tests/fields_illegal_tracing.txt

 In file included from fields_illegal_tracing.cpp:5:
 ./fields_illegal_tracing.h:32:1: warning: [blink-gc] Class 'PartObject' contains invalid fields.
 class PartObject {
 ^
 ./fields_illegal_tracing.h:37:5: note: [blink-gc] OwnPtr field 'm_obj1' to a GC managed class declared here:
     OwnPtr<HeapObject> m_obj1;
     ^
 ./fields_illegal_tracing.h:38:5: note: [blink-gc] RefPtr field 'm_obj2' to a GC managed class declared here:
     RefPtr<HeapObject> m_obj2;
     ^
 ./fields_illegal_tracing.h:40:5: note: [blink-gc] std::unique_ptr field 'm_obj4' to a GC managed class declared here:
     std::unique_ptr<HeapObject> m_obj4;
     ^
-./fields_illegal_tracing.h:43:1: warning: [blink-gc] Class 'HeapObject' contains invalid fields.
+./fields_illegal_tracing.h:42:5: note: [blink-gc] Iterator field 'm_iterator2' to a GC managed collection declared here:
+    HeapVector<Member<HeapObject>>::iterator m_iterator2;
+    ^
+./fields_illegal_tracing.h:43:5: note: [blink-gc] Iterator field 'm_iterator3' to a GC managed collection declared here:
+    HeapHashSet<PartObject>::const_iterator m_iterator3;
+    ^
+./fields_illegal_tracing.h:46:1: warning: [blink-gc] Class 'HeapObject' contains invalid fields.
 class HeapObject : public GarbageCollectedFinalized<HeapObject> {
 ^
-./fields_illegal_tracing.h:48:5: note: [blink-gc] OwnPtr field 'm_obj1' to a GC managed class declared here:
+./fields_illegal_tracing.h:51:5: note: [blink-gc] OwnPtr field 'm_obj1' to a GC managed class declared here:
     OwnPtr<HeapObject> m_obj1;
     ^
-./fields_illegal_tracing.h:49:5: note: [blink-gc] RefPtr field 'm_obj2' to a GC managed class declared here:
+./fields_illegal_tracing.h:52:5: note: [blink-gc] RefPtr field 'm_obj2' to a GC managed class declared here:
     RefPtr<HeapObject> m_obj2;
     ^
-./fields_illegal_tracing.h:51:5: note: [blink-gc] std::unique_ptr field 'm_obj4' to a GC managed class declared here:
+./fields_illegal_tracing.h:54:5: note: [blink-gc] std::unique_ptr field 'm_obj4' to a GC managed class declared here:
     std::unique_ptr<HeapObject> m_obj4;
     ^
+./fields_illegal_tracing.h:55:5: note: [blink-gc] Iterator field 'm_iterator3' to a GC managed collection declared here:
+    HeapHashMap<int, Member<HeapObject>>::reverse_iterator m_iterator3;
+    ^
+./fields_illegal_tracing.h:56:5: note: [blink-gc] Iterator field 'm_iterator4' to a GC managed collection declared here:
+    HeapDeque<Member<HeapObject>>::const_reverse_iterator m_iterator4;
+    ^
+./fields_illegal_tracing.h:58:5: note: [blink-gc] Iterator field 'm_iterator6' to a GC managed collection declared here:
+    HeapLinkedHashSet<Member<HeapObject>>::const_iterator m_iterator6;
+    ^
 fields_illegal_tracing.cpp:9:1: warning: [blink-gc] Class 'PartObject' has untraced or not traceable fields.
 void PartObject::trace(Visitor* visitor) {
 ^
 ./fields_illegal_tracing.h:37:5: note: [blink-gc] Untraceable field 'm_obj1' declared here:
     OwnPtr<HeapObject> m_obj1;
     ^
 ./fields_illegal_tracing.h:38:5: note: [blink-gc] Untraceable field 'm_obj2' declared here:
     RefPtr<HeapObject> m_obj2;
     ^
 ./fields_illegal_tracing.h:40:5: note: [blink-gc] Untraceable field 'm_obj4' declared here:
     std::unique_ptr<HeapObject> m_obj4;
     ^
 fields_illegal_tracing.cpp:16:1: warning: [blink-gc] Class 'HeapObject' has untraced or not traceable fields.
 void HeapObject::trace(Visitor* visitor) {
 ^
-./fields_illegal_tracing.h:48:5: note: [blink-gc] Untraceable field 'm_obj1' declared here:
+./fields_illegal_tracing.h:51:5: note: [blink-gc] Untraceable field 'm_obj1' declared here:
     OwnPtr<HeapObject> m_obj1;
     ^
-./fields_illegal_tracing.h:49:5: note: [blink-gc] Untraceable field 'm_obj2' declared here:
+./fields_illegal_tracing.h:52:5: note: [blink-gc] Untraceable field 'm_obj2' declared here:
     RefPtr<HeapObject> m_obj2;
     ^
-./fields_illegal_tracing.h:51:5: note: [blink-gc] Untraceable field 'm_obj4' declared here:
+./fields_illegal_tracing.h:54:5: note: [blink-gc] Untraceable field 'm_obj4' declared here:
     std::unique_ptr<HeapObject> m_obj4;
     ^
+./fields_illegal_tracing.h:57:5: note: [blink-gc] Untraced field 'm_iterator5' declared here:
+    HeapListHashSet<Member<HeapObject>>::const_iterator m_iterator5;
+    ^
 4 warnings generated.