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

content/browser/BUILD.gn

Index: content/browser/BUILD.gn
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index 6a325553e8b7b23216774f351608c052d75ed548..24186c83f5cd3594b348a6d9bb744c9132112adb 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -4,6 +4,7 @@
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
+import("//gpu/vulkan/features.gni")
 import("//media/media_options.gni")
 import("//ppapi/features/features.gni")
 import("//printing/features/features.gni")
@@ -83,6 +84,7 @@ source_set("browser") {
     "//gpu/command_buffer/client:gles2_implementation",
     "//gpu/command_buffer/client:gles2_interface",
     "//gpu/ipc/host",
+    "//gpu/vulkan:features",
     "//ipc",
     "//media",
     "//media/capture",