Fix behavior of sk_sp::reset() and add unittest.

include/core/SkRefCnt.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkRefCnt_DEFINED
 #define SkRefCnt_DEFINED
 
@@ skipping 320 matching lines @@
     bool operator!() const { return this->get() == nullptr; }
 
     T* get() const { return fPtr; }
     T* operator->() const { return fPtr; }
 
     /**
      *  Adopt the new bare pointer, and call unref() on any previously held object (if not null).
      *  No call to ref() will be made.
      */
     void reset(T* ptr = nullptr) {
-        if (fPtr != ptr) {
-            SkSafeUnref(fPtr);
-            fPtr = ptr;
-        }
+        SkSafeUnref(fPtr);
+        fPtr = ptr;
     }
 
     /**
      *  Return the bare pointer, and set the internal object pointer to nullptr.
      *  The caller must assume ownership of the object, and manage its reference count directly.
      *  No call to unref() will be made.
      */
     T* SK_WARN_UNUSED_RESULT release() {
         T* ptr = fPtr;
         fPtr = nullptr;
         return ptr;
     }
 
 private:
     T*  fPtr;
 };
 
 template <typename T, typename... Args>
 sk_sp<T> sk_make_sp(Args&&... args) {
     return sk_sp<T>(new T(std::forward<Args>(args)...));
 }
 
 #endif