Blink GC plugin: disallow own pointers to GC allocated collections.

tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp

Index: tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
diff --git a/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp b/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
index 0ea8f4cac36330f042492810e86e52989a56a7ab..d1d3a468ec88b1af5cdd8d145716e059d8239862 100644
--- a/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
+++ b/tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp
@@ -486,7 +486,7 @@ class CheckFieldsVisitor : public RecursiveEdgeVisitor {
     return !invalid_fields_.empty();
   }
 
-  void VisitMember(Member* edge) override {
+  void AtMember(Member* edge) override {
     if (managed_host_)
       return;
     // A member is allowed to appear in the context of a root.
@@ -499,7 +499,7 @@ class CheckFieldsVisitor : public RecursiveEdgeVisitor {
     invalid_fields_.push_back(std::make_pair(current_, kMemberInUnmanaged));
   }
 
-  void VisitValue(Value* edge) override {
+  void AtValue(Value* edge) override {
     // TODO: what should we do to check unions?
     if (edge->value()->record()->isUnion())
       return;
@@ -540,6 +540,11 @@ class CheckFieldsVisitor : public RecursiveEdgeVisitor {
     }
   }
 
+  void AtCollection(Collection* edge) override {
+    if (edge->on_heap() && Parent() && Parent()->IsOwnPtr())
+      invalid_fields_.push_back(std::make_pair(current_, kOwnPtrToGCManaged));
+  }
+
  private:
   Error InvalidSmartPtr(Edge* ptr) {
     if (ptr->IsRawPtr())