[Impeller] Added helper messaging around validation usage (#42708)

Adds some helper messages around using validation layers.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: gaaclarke

impeller/renderer/backend/vulkan/BUILD.gn

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//flutter/vulkan/config.gni")
 import("../../../tools/impeller.gni")
 
 impeller_component("vulkan_unittests") {
@@ -96,4 +97,8 @@ impeller_component("vulkan") {
     "//third_party/vulkan-deps/vulkan-headers/src:vulkan_headers",
     "//third_party/vulkan_memory_allocator",
   ]
+
+  if (enable_vulkan_validation_layers) {
+    defines = [ "IMPELLER_ENABLE_VULKAN_VALIDATION_LAYERS" ]
+  }
 }

impeller/renderer/backend/vulkan/context_vk.cc

@@ -184,6 +184,17 @@ void ContextVK::Setup(Settings settings) {
 
   vk::InstanceCreateInfo instance_info;
   if (caps->AreValidationsEnabled()) {
+    std::stringstream ss;
+    ss << "Enabling validation layers, features: [";
+    for (const auto& validation : enabled_validations) {
+      ss << vk::to_string(validation) << " ";
+    }
+    ss << "]";
+    FML_LOG(ERROR) << ss.str();
+#if !defined(IMPELLER_ENABLE_VULKAN_VALIDATION_LAYERS) && FML_OS_ANDROID
+    FML_LOG(ERROR) << "Vulkan validation layers turned on but the gn argument "
+                      "`--enable-vulkan-validation-layers` is missing.";
+#endif
     instance_info.pNext = &validation;
   }
   instance_info.setPEnabledLayerNames(enabled_layers_c);