Remove enable_vulkan from build/config, and move it to the buildflag_header system.

cc/output/vulkan_in_process_context_provider.h

Index: cc/output/vulkan_in_process_context_provider.h
diff --git a/cc/output/vulkan_in_process_context_provider.h b/cc/output/vulkan_in_process_context_provider.h
index c477f2f3eee20c233393ce79346f8d52188a52c9..2cca29772cfbeeabe3c5df22b67204ba44244382 100644
--- a/cc/output/vulkan_in_process_context_provider.h
+++ b/cc/output/vulkan_in_process_context_provider.h
@@ -9,6 +9,7 @@
 
 #include "cc/base/cc_export.h"
 #include "cc/output/vulkan_context_provider.h"
+#include "gpu/vulkan/features.h"
 
 namespace gpu {
 class VulkanDeviceQueue;
@@ -31,7 +32,7 @@ class CC_EXPORT VulkanInProcessContextProvider : public VulkanContextProvider {
   ~VulkanInProcessContextProvider() override;
 
  private:
-#if defined(ENABLE_VULKAN)
+#if BUILDFLAG(ENABLE_VULKAN)
   std::unique_ptr<gpu::VulkanDeviceQueue> device_queue_;
 #endif