Implement RefCountedCopyable<T>.

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

Index: third_party/WebKit/Source/platform/wtf/RefCounted.h
diff --git a/third_party/WebKit/Source/platform/wtf/RefCounted.h b/third_party/WebKit/Source/platform/wtf/RefCounted.h
index 357e294e9ac2ebbf1b355cc128c920eb111183d9..d6c6776c6f0101cb274f99705f82bce6b56c049b 100644
--- a/third_party/WebKit/Source/platform/wtf/RefCounted.h
+++ b/third_party/WebKit/Source/platform/wtf/RefCounted.h
@@ -164,8 +164,17 @@ class RefCounted : public RefCountedBase {
 #endif
 };
 
+// Allows subclasses to use the default copy constructor.
+template <typename T>
+class RefCountedCopyable : public RefCounted<T> {
+ protected:
+  RefCountedCopyable() = default;
+  RefCountedCopyable(const RefCountedCopyable&) : RefCounted<T>() {}
+};
+
 }  // namespace WTF
 
 using WTF::RefCounted;
+using WTF::RefCountedCopyable;
 
 #endif  // RefCounted_h