BUILD.gn: Make the vulkan-headers directory overridable.

This will allow embedders to move the place of that directory. Also
removes a deprecated proxy target that's no longer used.

Bug: tint:700
Bug: chromium:1195554

Change-Id: I26b0632f8e8d06b9322dbd731775971eb9280e8e
diff --git a/BUILD.gn b/BUILD.gn
index 495bf9c..fb1a120 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -4,6 +4,10 @@
 #
 # Custom GN integration for VulkanMemoryAllocator.
 
+declare_args() {
+  vma_vulkan_headers_dir = "//third_party/vulkan-deps/vulkan-headers/src"
+}
+
 config("vulkan_memory_allocator_config") {
   include_dirs = [ "src" ]
   if (is_clang) {
@@ -32,12 +36,6 @@
     "src/vk_mem_alloc.h",
     "vma.cpp",
   ]
-  deps = [ "//third_party/vulkan-deps/vulkan-headers/src:vulkan_headers" ]
+  deps = [ "${vma_vulkan_headers_dir}:vulkan_headers" ]
   public_configs = [ ":vulkan_memory_allocator_config" ]
 }
-
-# TODO: remove this target when chrome and ANGLE are updated to use
-# vulkan_memory_allocator
-source_set("vulkan_memory_allocator_with_usage") {
-  public_deps = [ ":vulkan_memory_allocator" ]
-}