blink_gc_plugin: report illegal on-heap iterators as warnings/errors.

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:42:5: note: [blink-gc] Iterator field 'm_iterator2' to a GC managed collection declared here:
+./fields_illegal_tracing.h:42:5: warning: [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:
+./fields_illegal_tracing.h:43:5: warning: [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:51:5: note: [blink-gc] OwnPtr field 'm_obj1' to a GC managed class declared here:
     OwnPtr<HeapObject> m_obj1;
     ^
 ./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: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:
+./fields_illegal_tracing.h:55:5: warning: [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:
+./fields_illegal_tracing.h:56:5: warning: [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:
+./fields_illegal_tracing.h:58:5: warning: [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:51:5: note: [blink-gc] Untraceable field 'm_obj1' declared here:
     OwnPtr<HeapObject> m_obj1;
     ^
 ./fields_illegal_tracing.h:52:5: note: [blink-gc] Untraceable field 'm_obj2' declared here:
     RefPtr<HeapObject> m_obj2;
     ^
 ./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.
+9 warnings generated.