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..671e49b2943759692f321f4c62f06f0bcabaf919 100644
--- a/third_party/WebKit/Source/platform/wtf/RefCounted.h
+++ b/third_party/WebKit/Source/platform/wtf/RefCounted.h
@@ -164,8 +164,19 @@ class RefCounted : public RefCountedBase {
 #endif
 };
 
+// Unlike subclasses of RefCounted where the copy constructor must be
+// handwritten, subclasses of RefCountedCopyable can use the default
+// compiler-generated copy constructor.

[alancutter (OOO until 2018), 2017/04/03 12:07:45]

This wording is a bit too fancy. For speed of reading the point should be the
first thing conveyed. "Allows subclasses to use their 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