Introduce GN variables for Vulkan repos.

This will allow them to be seamlessly overrideen in other repos.

Bug: angleproject:5390
Change-Id: I973cae48b5683e39ea3b07898b95956511260319
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2591107
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 2589ee3..9b4e382 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -402,7 +402,7 @@
 
   if (!is_chromeos && angle_has_build) {
     sources += libangle_gpu_info_util_vulkan_sources
-    deps += [ "$angle_root/third_party/vulkan-headers/src:vulkan_headers" ]
+    deps += [ "$angle_vulkan_headers_dir:vulkan_headers" ]
   }
 
   if (is_android) {