Enable WTF::HashSet<OwnPtr<> >

Source/wtf/OwnPtr.h

Index: Source/wtf/OwnPtr.h
diff --git a/Source/wtf/OwnPtr.h b/Source/wtf/OwnPtr.h
index e0dde828cec1773f281a3f33752b0897aefeed10..a923b152973ddda9597db4594670a1df1ea89619 100644
--- a/Source/wtf/OwnPtr.h
+++ b/Source/wtf/OwnPtr.h
@@ -22,6 +22,7 @@
 #ifndef WTF_OwnPtr_h
 #define WTF_OwnPtr_h
 
+#include "wtf/HashTableDeletedValueType.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/NullPtr.h"
 #include "wtf/OwnPtrCommon.h"
@@ -49,6 +50,10 @@ namespace WTF {
         OwnPtr(const PassOwnPtr<T>&);
         template<typename U> OwnPtr(const PassOwnPtr<U>&, EnsurePtrConvertibleArgDecl(U, T));
 
+        // Hash table deleted values, which are only constructed and never copied or destroyed.
+        OwnPtr(HashTableDeletedValueType) : m_ptr(hashTableDeletedValue()) { }
+        bool isHashTableDeletedValue() const { return m_ptr == hashTableDeletedValue(); }
+
 #if !COMPILER_SUPPORTS(CXX_RVALUE_REFERENCES)
         // This copy constructor is used implicitly by gcc when it generates
         // transients for assigning a PassOwnPtr<T> object to a stack-allocated
@@ -94,6 +99,8 @@ namespace WTF {
 
         void swap(OwnPtr& o) { std::swap(m_ptr, o.m_ptr); }
 
+        static T* hashTableDeletedValue() { return reinterpret_cast<T*>(-1); }
+
     private:
 #if !COMPILER_SUPPORTS(CXX_RVALUE_REFERENCES)
         // If rvalue references are supported, noncopyable takes care of this.