Revert of GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/Edge.h

Index: tools/clang/blink_gc_plugin/Edge.h
diff --git a/tools/clang/blink_gc_plugin/Edge.h b/tools/clang/blink_gc_plugin/Edge.h
index 79aa409ca1db5cea3c804aeaa8e9f89efe85e6c9..91e569867d8fe0ec448a9eb4acb52a5287ae3522 100644
--- a/tools/clang/blink_gc_plugin/Edge.h
+++ b/tools/clang/blink_gc_plugin/Edge.h
@@ -14,16 +14,14 @@
 class RecordInfo;
 
 class Edge;
-class Collection;
-class CrossThreadPersistent;
-class Member;
-class OwnPtr;
-class Persistent;
+class Value;
 class RawPtr;
 class RefPtr;
-class UniquePtr;
-class Value;
+class OwnPtr;
+class Member;
 class WeakMember;
+class Persistent;
+class Collection;
 
 // Bare-bones visitor.
 class EdgeVisitor {
@@ -33,11 +31,9 @@
   virtual void VisitRawPtr(RawPtr*) {}
   virtual void VisitRefPtr(RefPtr*) {}
   virtual void VisitOwnPtr(OwnPtr*) {}
-  virtual void VisitUniquePtr(UniquePtr*) {}
   virtual void VisitMember(Member*) {}
   virtual void VisitWeakMember(WeakMember*) {}
   virtual void VisitPersistent(Persistent*) {}
-  virtual void VisitCrossThreadPersistent(CrossThreadPersistent*) {}
   virtual void VisitCollection(Collection*) {}
 };
 
@@ -49,11 +45,9 @@
   void VisitRawPtr(RawPtr*) override;
   void VisitRefPtr(RefPtr*) override;
   void VisitOwnPtr(OwnPtr*) override;
-  void VisitUniquePtr(UniquePtr*) override;
   void VisitMember(Member*) override;
   void VisitWeakMember(WeakMember*) override;
   void VisitPersistent(Persistent*) override;
-  void VisitCrossThreadPersistent(CrossThreadPersistent*) override;
   void VisitCollection(Collection*) override;
 
  protected:
@@ -68,11 +62,9 @@
   virtual void AtRawPtr(RawPtr*);
   virtual void AtRefPtr(RefPtr*);
   virtual void AtOwnPtr(OwnPtr*);
-  virtual void AtUniquePtr(UniquePtr*);
   virtual void AtMember(Member*);
   virtual void AtWeakMember(WeakMember*);
   virtual void AtPersistent(Persistent*);
-  virtual void AtCrossThreadPersistent(CrossThreadPersistent*);
   virtual void AtCollection(Collection*);
 
  private:
@@ -97,9 +89,9 @@
   virtual bool IsRawPtr() { return false; }
   virtual bool IsRefPtr() { return false; }
   virtual bool IsOwnPtr() { return false; }
-  virtual bool IsUniquePtr() { return false; }
   virtual bool IsMember() { return false; }
   virtual bool IsWeakMember() { return false; }
+  virtual bool IsPersistent() { return false; }
   virtual bool IsCollection() { return false; }
 };
 
@@ -143,7 +135,7 @@
   LivenessKind Kind() { return kWeak; }
   bool NeedsFinalization() { return false; }
   TracingStatus NeedsTracing(NeedsTracingOption) {
-    return TracingStatus::Illegal();
+    return TracingStatus::Unneeded();
   }
   void Accept(EdgeVisitor* visitor) { visitor->VisitRawPtr(this); }
 
@@ -159,7 +151,7 @@
   LivenessKind Kind() { return kStrong; }
   bool NeedsFinalization() { return true; }
   TracingStatus NeedsTracing(NeedsTracingOption) {
-    return TracingStatus::Illegal();
+    return TracingStatus::Unneeded();
   }
   void Accept(EdgeVisitor* visitor) { visitor->VisitRefPtr(this); }
 };
@@ -171,21 +163,9 @@
   LivenessKind Kind() { return kStrong; }
   bool NeedsFinalization() { return true; }
   TracingStatus NeedsTracing(NeedsTracingOption) {
-    return TracingStatus::Illegal();
+    return TracingStatus::Unneeded();
   }
   void Accept(EdgeVisitor* visitor) { visitor->VisitOwnPtr(this); }
-};
-
-class UniquePtr : public PtrEdge {
- public:
-  explicit UniquePtr(Edge* ptr) : PtrEdge(ptr) { }
-  bool IsUniquePtr() { return true; }
-  LivenessKind Kind() { return kStrong; }
-  bool NeedsFinalization() { return true; }
-  TracingStatus NeedsTracing(NeedsTracingOption) {
-    return TracingStatus::Illegal();
-  }
-  void Accept(EdgeVisitor* visitor) { visitor->VisitUniquePtr(this); }
 };
 
 class Member : public PtrEdge {
@@ -215,25 +195,13 @@
 class Persistent : public PtrEdge {
  public:
   explicit Persistent(Edge* ptr) : PtrEdge(ptr) { }
+  bool IsPersistent() { return true; }
   LivenessKind Kind() { return kRoot; }
   bool NeedsFinalization() { return true; }
   TracingStatus NeedsTracing(NeedsTracingOption) {
     return TracingStatus::Unneeded();
   }
   void Accept(EdgeVisitor* visitor) { visitor->VisitPersistent(this); }
-};
-
-class CrossThreadPersistent : public PtrEdge {
- public:
-  explicit CrossThreadPersistent(Edge* ptr) : PtrEdge(ptr) { }
-  LivenessKind Kind() { return kRoot; }
-  bool NeedsFinalization() { return true; }
-  TracingStatus NeedsTracing(NeedsTracingOption) {
-    return TracingStatus::Illegal();
-  }
-  void Accept(EdgeVisitor* visitor) {
-    visitor->VisitCrossThreadPersistent(this);
-  }
 };
 
 class Collection : public Edge {