Replace some nullptrs with VK_NULL_HANDLE

src/gpu/vk/GrVkImage.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrVkImage_DEFINED
 #define GrVkImage_DEFINED
 
@@ skipping 11 matching lines @@
     public:
         enum Flags {
             kNo_Flags = 0,
             kLinearTiling_Flag = 0x01
         };
 
         VkImage        fImage;
         VkDeviceMemory fAlloc;
         Flags          fFlags;
 
-        Resource() : INHERITED(), fImage(nullptr), fAlloc(nullptr), fFlags(kNo_Flags) {}
+        Resource() : INHERITED(), fImage(VK_NULL_HANDLE), fAlloc(VK_NULL_HANDLE), fFlags(kNo_Flags) {}
 
         Resource(VkImage image, VkDeviceMemory alloc, Flags flags)
             : fImage(image), fAlloc(alloc), fFlags(flags) {}
 
         ~Resource() override {}
     private:
         void freeGPUData(const GrVkGpu* gpu) const override;
 
         typedef GrVkResource INHERITED;
     };
@@ skipping 56 matching lines @@
     void releaseImage(const GrVkGpu* gpu);
     void abandonImage();
 
     const Resource* fResource;
 
     VkImageLayout   fCurrentLayout;
 
 };
 
 #endif