sk_sp: fix contravariant constructors

tests/RefCntTest.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkRefCnt.h"
 #include "SkThreadUtils.h"
 #include "SkTypes.h"
@@ skipping 256 matching lines @@
     baz = EffectImpl::Create();
     check(reporter, 0, 0, 1, 0);
     paint.set(baz);
     check(reporter, 1, 0, 1, 0);
     baz = nullptr;
     check(reporter, 1, 1, 1, 0);
     paint.set(nullptr);
     check(reporter, 1, 2, 1, 1);
 }
 
+namespace {
+struct FooAbstract : public SkRefCnt {
+    virtual void f() = 0;
+};
+struct FooConcrete : public FooAbstract {
+    void f() override {}
+};
+}
+static sk_sp<FooAbstract> make_foo() {
+    // can not cast FooConcrete to FooAbstract.
+    // can cast FooConcrete* to FooAbstract*.
+    return sk_make_sp<FooConcrete>();
+}
+DEF_TEST(sk_make_sp, r) {
+    auto x = make_foo();
+}