blink_gc_plugin: disallow WeakMember<> fields in off-heap objects.

tools/clang/blink_gc_plugin/tests/member_in_offheap_class.txt

Index: tools/clang/blink_gc_plugin/tests/member_in_offheap_class.txt
diff --git a/tools/clang/blink_gc_plugin/tests/member_in_offheap_class.txt b/tools/clang/blink_gc_plugin/tests/member_in_offheap_class.txt
index 9d5f2388dc8b3d140c87d7b6cbbef915658db8ff..090778a8ff16189719977a6389d7360b6299c892 100644
--- a/tools/clang/blink_gc_plugin/tests/member_in_offheap_class.txt
+++ b/tools/clang/blink_gc_plugin/tests/member_in_offheap_class.txt
@@ -5,13 +5,16 @@ class OffHeapObject {
 ./member_in_offheap_class.h:18:5: note: [blink-gc] Member field 'm_obj' in unmanaged class declared here:
     Member<HeapObject> m_obj; // Must not contain Member.
     ^
-./member_in_offheap_class.h:22:1: warning: [blink-gc] Class 'StackObject' contains invalid fields.
+./member_in_offheap_class.h:19:5: note: [blink-gc] Member field 'm_weak' in unmanaged class declared here:
+    WeakMember<HeapObject> m_weak;  // Must not contain WeakMember.
+    ^
+./member_in_offheap_class.h:23:1: warning: [blink-gc] Class 'StackObject' contains invalid fields.
 class StackObject {
 ^
-./member_in_offheap_class.h:26:5: note: [blink-gc] Member field 'm_memberOff' to non-GC managed class declared here:
+./member_in_offheap_class.h:27:5: note: [blink-gc] Member field 'm_memberOff' to non-GC managed class declared here:
     Member<OffHeapObject> m_memberOff; // NOT OK
     ^
-./member_in_offheap_class.h:27:5: note: [blink-gc] Member field 'm_heapVectorMemberOff' to non-GC managed class declared here:
+./member_in_offheap_class.h:28:5: note: [blink-gc] Member field 'm_heapVectorMemberOff' to non-GC managed class declared here:
     HeapVector<Member<OffHeapObject>> m_heapVectorMemberOff; // NOT OK
     ^
 2 warnings generated.