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 8f7feccc9c49c61dec056168f4bae3933a03a4ce..fff4a781659e159ca1e7c2a7975a8e7ab30e312f 100644
--- a/tools/clang/blink_gc_plugin/tests/heap/stubs.h
+++ b/tools/clang/blink_gc_plugin/tests/heap/stubs.h
@@ -118,6 +118,23 @@ class HashMap {};
 
 }
 
+// Empty namespace declaration to exercise internal
+// handling of namespace equality.
+namespace std {
+  /* empty */
+}
+
+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 +195,27 @@ 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;