vulkan: Don't enable VK_LAYER_LUNARG_standard_validation under X11

Under X11 we use the VK_KHR_xlib_surface extension but this extension is
not supported by VK_LAYER_LUNARG_standard_validation. When these
extensions are used together, the validation layer never observes the
surface creation and fails the first time it is used.

Disable VK_LAYER_LUNARG_standard_validation so that debug builds can be
used with vulkan under X11.

Bug: 843346
Test: vulkan_tests with debug build

Change-Id: I4a42a8adb134a62043a14d416ae30f02e254cbd6
Reviewed-on: https://chromium-review.googlesource.com/1060459Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558874}

gpu/vulkan/vulkan_implementation.cc

@@ -110,7 +110,11 @@ struct VulkanInstance {
     }
 
     std::unordered_set<std::string> desired_layers({
+#if !defined(VK_USE_PLATFORM_XLIB_KHR)
+      // TODO(crbug.com/843346): Make validation work in combination with
+      // VK_KHR_xlib_surface or switch to VK_KHR_xcb_surface.
       "VK_LAYER_LUNARG_standard_validation",
+#endif
     });
 
     for (const VkLayerProperties& layer_property : instance_layers) {