message: Fix link in standalone on Linux
  There were some missing DAWN_NATIVE_EXPORT arributes for new structures
  in VulkanBackend.h which resulted in linking errors.
diff: @@ -33,7 +33,7 @@ namespace dawn_native { namespace vulkan {
  };
  // Common properties of external images
  - struct ExternalImageDescriptor {
  + struct DAWN_NATIVE_EXPORT ExternalImageDescriptor {
  public:
  const ExternalImageDescriptorType type; // Must match the subclass
  const WGPUTextureDescriptor* cTextureDescriptor; // Must match image creation params
  @@ -55,7 +55,7 @@ namespace dawn_native { namespace vulkan {
  // Can't use DAWN_PLATFORM_LINUX since header included in both dawn and chrome
  #ifdef __linux__
  // Common properties of external images represented by FDs
  - struct ExternalImageDescriptorFD : ExternalImageDescriptor {
  + struct DAWN_NATIVE_EXPORT ExternalImageDescriptorFD : ExternalImageDescriptor {
  public:
  int memoryFD; // A file descriptor from an export of the memory of the image
  std::vector<int> waitFDs; // File descriptors of semaphores which will be waited on
  @@ -65,7 +65,7 @@ namespace dawn_native { namespace vulkan {
  };
  // Descriptor for opaque file descriptor image import
  - struct ExternalImageDescriptorOpaqueFD : ExternalImageDescriptorFD {
  + struct DAWN_NATIVE_EXPORT ExternalImageDescriptorOpaqueFD : ExternalImageDescriptorFD {
  ExternalImageDescriptorOpaqueFD();
  VkDeviceSize allocationSize; // Must match VkMemoryAllocateInfo from image creation
  @@ -73,7 +73,7 @@ namespace dawn_native { namespace vulkan {
  };
  // Descriptor for dma-buf file descriptor image import
  - struct ExternalImageDescriptorDmaBuf : ExternalImageDescriptorFD {
  + struct DAWN_NATIVE_EXPORT ExternalImageDescriptorDmaBuf : ExternalImageDescriptorFD {
  ExternalImageDescriptorDmaBuf();
  uint32_t stride; // Stride of the buffer in bytes
  