GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/tests/heap/stubs.h

Index: tools/clang/blink_gc_plugin/tests/heap/stubs.h
diff --git a/tools/clang/blink_gc_plugin/tests/heap/stubs.h b/tools/clang/blink_gc_plugin/tests/heap/stubs.h
index a31ef3abeed309974308f1e39bfa080680bfea07..f27bf9ee2ecd74e0e5ec1264b7cf896347a932aa 100644
--- a/tools/clang/blink_gc_plugin/tests/heap/stubs.h
+++ b/tools/clang/blink_gc_plugin/tests/heap/stubs.h
@@ -118,6 +118,17 @@ class HashMap {};
 
 }
 
+namespace std {
+
+template<typename T> class unique_ptr {
+public:
+    ~unique_ptr() { }
+    operator T*() const { return 0; }
+    T* operator->() { return 0; }
+};
+
+}
+
 namespace blink {
 
 using namespace WTF;
@@ -178,6 +189,27 @@ public:
     bool operator!() const { return false; }
 };
 
+template<typename T> class WeakPersistent {
+public:
+    operator T*() const { return 0; }
+    T* operator->() { return 0; }
+    bool operator!() const { return false; }
+};
+
+template<typename T> class CrossThreadPersistent {
+public:
+    operator T*() const { return 0; }
+    T* operator->() { return 0; }
+    bool operator!() const { return false; }
+};
+
+template<typename T> class CrossThreadWeakPersistent {
+public:
+    operator T*() const { return 0; }
+    T* operator->() { return 0; }
+    bool operator!() const { return false; }
+};
+
 class HeapAllocator {
 public:
     static const bool isGarbageCollected = true;