Move CrossThreadCopier from platform/ to wtf/

third_party/WebKit/Source/wtf/RefPtr.h

Index: third_party/WebKit/Source/wtf/RefPtr.h
diff --git a/third_party/WebKit/Source/wtf/RefPtr.h b/third_party/WebKit/Source/wtf/RefPtr.h
index 0097b914fe56229596f906da298c79dc80788251..412ec96b97ef01c30c2d63b4206d72e9c5ec935b 100644
--- a/third_party/WebKit/Source/wtf/RefPtr.h
+++ b/third_party/WebKit/Source/wtf/RefPtr.h
@@ -24,11 +24,16 @@
 #define WTF_RefPtr_h
 
 #include "wtf/Allocator.h"
+#include "wtf/CrossThreadCopier.h"
 #include "wtf/HashTableDeletedValueType.h"
 #include "wtf/PassRefPtr.h"
+#include "wtf/ThreadSafeRefCounted.h"
+#include "wtf/TypeTraits.h"
 #include <algorithm>
 #include <utility>
 
+class SkRefCnt;
+
 namespace WTF {
 
 template <typename T> class PassRefPtr;
@@ -86,6 +91,12 @@ private:
 };
 
 template <typename T>
+struct CrossThreadCopier<RefPtr<T>> : public CrossThreadCopierPassThrough<RefPtr<T>> {
+    STATIC_ONLY(CrossThreadCopier);
+    static_assert(IsSubclassOfTemplate<T, ThreadSafeRefCounted>::value || std::is_base_of<SkRefCnt, T>::value, "RefPtr<T> can be passed across threads only if T is ThreadSafeRefCounted or SkRefCnt.");
+};
+
+template <typename T>
 template <typename U> inline RefPtr<T>::RefPtr(const PassRefPtr<U>& o, EnsurePtrConvertibleArgDefn(U, T))
     : m_ptr(o.leakRef())
 {