message: Vulkan: add macro for vktrace
  Hide vktrace layout behide a marco 'DAWN_USE_VKTRACE' and enable it in
  build if need.
diff: @@ -421,13 +421,16 @@ namespace dawn_native { namespace vulkan {
  extensions->push_back(extension);
  };
  - // vktrace works by instering a layer, so we need to explicitly enable it if it is present.
  + // vktrace works by instering a layer, but we hide it behind a macro unless we are
  + // debugging in vktrace.
  // Also it is good to put it in first position so that it doesn't see Vulkan calls inserted
  // by other layers.
  +#if defined(DAWN_USE_VKTRACE)
  if (mGlobalInfo.vktrace) {
  layersToRequest.push_back(kLayerNameLunargVKTrace);
  usedKnobs->vktrace = true;
  }
  +#endif
  // RenderDoc installs a layer at the system level for its capture but we don't want to use
  // it unless we are debugging in RenderDoc so we hide it behind a macro.
  #if defined(DAWN_USE_RENDERDOC)
  