Initialize pNext in VkImageMemoryBarrier

According to manual, it must be either NULL or pointer to an
extension-specific structure. Also GCC barks on skipping fields in
initialisers (8 times as 8 fields follow skipped pNext):

../../gpu/vulkan/vulkan_swap_chain.cc:94:3: sorry, unimplemented: non-trivial designated initializers not supported

Change-Id: I744b9dc84c24b927ec1172f4044f9ca7fbf64029
Reviewed-on: https://chromium-review.googlesource.com/1226600Reviewed-by: Peng Huang <penghuang@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594828}

gpu/vulkan/vulkan_swap_chain.cc

@@ -76,6 +76,7 @@ void CmdSetImageLayout(VulkanCommandBuffer* command_buffer,
   DCHECK_NE(layout, old_layout);
   VkImageMemoryBarrier image_memory_barrier = {
       .sType = VK_STRUCTURE_TYPE_IMAGE_MEMORY_BARRIER,
+      .pNext = nullptr,
       .srcAccessMask = GetAccessMask(old_layout),
       .dstAccessMask = GetAccessMask(layout),
       .oldLayout = old_layout,