Fix type conversion compile errors

src/gpu/vk/GrVkGpu.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrVkGpu.h"
 
 #include "GrContextOptions.h"
@@ skipping 71 matching lines @@
 
 GrVkGpu::GrVkGpu(GrContext* context, const GrContextOptions& options,
                  const GrVkBackendContext* backendCtx)
     : INHERITED(context)
     , fDevice(backendCtx->fDevice)
     , fQueue(backendCtx->fQueue)
     , fResourceProvider(this) {
     fBackendContext.reset(backendCtx);
 
 #ifdef ENABLE_VK_LAYERS
-    fCallback = nullptr;
+    fCallback = VK_NULL_HANDLE;
     if (backendCtx->fExtensions & kEXT_debug_report_GrVkExtensionFlag) {
         // Setup callback creation information
         VkDebugReportCallbackCreateInfoEXT callbackCreateInfo;
         callbackCreateInfo.sType = VK_STRUCTURE_TYPE_DEBUG_REPORT_CREATE_INFO_EXT;
         callbackCreateInfo.pNext = nullptr;
         callbackCreateInfo.flags = VK_DEBUG_REPORT_ERROR_BIT_EXT |
                                    VK_DEBUG_REPORT_WARNING_BIT_EXT |
                                    //VK_DEBUG_REPORT_INFORMATION_BIT_EXT |
                                    //VK_DEBUG_REPORT_DEBUG_BIT_EXT |
                                    VK_DEBUG_REPORT_PERFORMANCE_WARNING_BIT_EXT;
@@ skipping 43 matching lines @@
     // must call this just before we destroy the VkDevice
     fResourceProvider.destroyResources();
 
     VK_CALL(DestroyCommandPool(fDevice, fCmdPool, nullptr));
 
     shaderc_compiler_release(fCompiler);
 
 #ifdef ENABLE_VK_LAYERS
     if (fCallback) {
         VK_CALL(DestroyDebugReportCallbackEXT(fBackendContext->fInstance, fCallback, nullptr));
-        fCallback = nullptr;
+        fCallback = VK_NULL_HANDLE;
     }
 #endif
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 void GrVkGpu::submitCommandBuffer(SyncQueue sync) {
     SkASSERT(fCurrentCmdBuffer);
     fCurrentCmdBuffer->end(this);
 
@@ skipping 1640 matching lines @@
     aglSwapBuffers(aglGetCurrentContext());
     int set_a_break_pt_here = 9;
     aglSwapBuffers(aglGetCurrentContext());
 #elif defined(SK_BUILD_FOR_WIN32)
     SwapBuf();
     int set_a_break_pt_here = 9;
     SwapBuf();
 #endif
 #endif
 }