Add external deleter support to WTF::RefCounted

NGPhysicalFragment needs special handling on the destruction
rather than being destroyed by WTF::RefCounted. However, existing
style of the destruction hook will be unavailable after unifying
base::RefCounted and WTF::RefCounted.

This CL adds a proper way to inject the destruction function of a
ref counted object. The destruction function will be injected to
base::RefCounted after the unification.

Bug: 763844
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: If763525a55c4621abd711a16a531817528a42b69
Reviewed-on: https://chromium-review.googlesource.com/681955Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504365}

third_party/WebKit/Source/core/layout/ng/ng_physical_fragment.cc

@@ -128,6 +128,11 @@ void AppendFragmentToString(const NGPhysicalFragment* fragment,
 
 }  // namespace
 
+// static
+void NGPhysicalFragmentTraits::Destruct(const NGPhysicalFragment* fragment) {
+  fragment->Destroy();
+}
+
 NGPhysicalFragment::NGPhysicalFragment(LayoutObject* layout_object,
                                        const ComputedStyle& style,
                                        NGPhysicalSize size,

third_party/WebKit/Source/core/layout/ng/ng_physical_fragment.h

@@ -19,6 +19,12 @@ class ComputedStyle;
 class LayoutObject;
 struct NGPixelSnappedPhysicalBoxStrut;
 
+class NGPhysicalFragment;
+
+struct CORE_EXPORT NGPhysicalFragmentTraits {
+  static void Destruct(const NGPhysicalFragment*);
+};
+
 // The NGPhysicalFragment contains the output geometry from layout. The
 // fragment stores all of its information in the physical coordinate system for
 // use by paint, hit-testing etc.
@@ -39,9 +45,10 @@ struct NGPixelSnappedPhysicalBoxStrut;
 //   (See https://drafts.csswg.org/css-backgrounds-3/#the-background-image)
 // - image (<img>, svg <image>) or video (<video>) elements that are
 //   placeholders for displaying them.
-class CORE_EXPORT NGPhysicalFragment : public RefCounted<NGPhysicalFragment>,
-                                       public DisplayItemClient,
-                                       public ImageResourceObserver {
+class CORE_EXPORT NGPhysicalFragment
+    : public RefCounted<NGPhysicalFragment, NGPhysicalFragmentTraits>,
+      public DisplayItemClient,
+      public ImageResourceObserver {
  public:
   enum NGFragmentType {
     kFragmentBox = 0,
@@ -133,13 +140,6 @@ class CORE_EXPORT NGPhysicalFragment : public RefCounted<NGPhysicalFragment>,
   void ShowFragmentTree() const;
 #endif
 
-  // Override RefCounted's deref() to ensure operator delete is called on the
-  // appropriate subclass type.
-  void Deref() const {
-    if (DerefBase())
-      Destroy();
-  }
-
  protected:
   NGPhysicalFragment(LayoutObject* layout_object,
                      const ComputedStyle& style,
@@ -162,6 +162,7 @@ class CORE_EXPORT NGPhysicalFragment : public RefCounted<NGPhysicalFragment>,
   unsigned border_edge_ : 4;  // NGBorderEdges::Physical
 
  private:
+  friend struct NGPhysicalFragmentTraits;
   void Destroy() const;
 };
 

third_party/WebKit/Source/platform/wtf/RefCounted.h

@@ -128,7 +128,17 @@ inline void Adopted(RefCountedBase* object) {
 }
 #endif
 
+template <typename T, typename Traits>
+class RefCounted;
+
 template <typename T>
+struct DefaultRefCountedTraits {
+  static void Destruct(const T* x) {
+    WTF::RefCounted<T, DefaultRefCountedTraits>::DeleteInternal(x);
+  }
+};
+
+template <typename T, typename Traits = DefaultRefCountedTraits<T>>
 class RefCounted : public RefCountedBase {
   WTF_MAKE_NONCOPYABLE(RefCounted);
 
@@ -140,11 +150,15 @@ class RefCounted : public RefCountedBase {
  public:
   void Deref() const {
     if (DerefBase())
-      delete static_cast<const T*>(this);
+      Traits::Destruct(static_cast<const T*>(this));
   }
 
  protected:
   RefCounted() {}
+
+ private:
+  friend struct DefaultRefCountedTraits<T>;
+  static void DeleteInternal(const T* x) { delete x; }
 };
 
 // Allows subclasses to use the default copy constructor.

third_party/WebKit/Source/platform/wtf/ThreadSafeRefCounted.h

@@ -55,16 +55,31 @@ class WTF_EXPORT ThreadSafeRefCountedBase {
   base::AtomicRefCount ref_count_;
 };
 
-template <class T>
+template <typename T, typename Traits>
+class ThreadSafeRefCounted;
+
+template <typename T>
+struct DefaultThreadSafeRefCountedTraits {
+  static void Destruct(const T* x) {
+    WTF::ThreadSafeRefCounted<
+        T, DefaultThreadSafeRefCountedTraits>::DeleteInternal(x);
+  }
+};
+
+template <typename T, typename Traits = DefaultThreadSafeRefCountedTraits<T>>
 class ThreadSafeRefCounted : public ThreadSafeRefCountedBase {
  public:
   void Deref() {
     if (DerefBase())
-      delete static_cast<T*>(this);
+      Traits::Destruct(static_cast<T*>(this));
   }
 
  protected:
   ThreadSafeRefCounted() {}
+
+ private:
+  friend struct DefaultThreadSafeRefCountedTraits<T>;
+  static void DeleteInternal(const T* x) { delete x; }
 };
 
 }  // namespace WTF

third_party/WebKit/public/platform/WebPrivatePtr.h

@@ -41,7 +41,7 @@
 #endif
 
 namespace WTF {
-template <class T>
+template <class T, typename Traits>
 class ThreadSafeRefCounted;
 }