Fix compile warning on MSVC-vulkan (VkDeviceSize -> size_t)

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2246183002

Review-Url: https://codereview.chromium.org/2246183002
diff --git a/src/gpu/vk/GrVkGpu.cpp b/src/gpu/vk/GrVkGpu.cpp
index f097aef..4f177ff 100644
--- a/src/gpu/vk/GrVkGpu.cpp
+++ b/src/gpu/vk/GrVkGpu.cpp
@@ -1056,7 +1056,8 @@
 
             VK_CALL(GetImageSubresourceLayout(fDevice, image, &subres, &layout));
 
-            if (!copy_testing_data(this, srcData, &alloc, rowCopyBytes, layout.rowPitch, h)) {
+            if (!copy_testing_data(this, srcData, &alloc, rowCopyBytes,
+                                   static_cast<size_t>(layout.rowPitch), h)) {
                 GrVkMemory::FreeImageMemory(this, linearTiling, alloc);
                 VK_CALL(DestroyImage(fDevice, image, nullptr));
                 return 0;