Add build_overrides/vulkan_memory_allocator.gni

After https://chromium-review.googlesource.com/c/external/github.com/GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator/+/2804453
the VulkanMemoryAllocator dependency will start including this config
file to get the position of the vulkan-headers.

Also factor common vulkan variables to vulkan_common.gni like is done
in Chromium.

Bug: tint:700
Bug: chromium:1195554

Change-Id: Idbb8b957c17b2cbdbd518c13706d02ef4d74e93f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/46862
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/build_overrides/vulkan_loader.gni b/build_overrides/vulkan_loader.gni
index 7185dea..ac6a61d 100644
--- a/build_overrides/vulkan_loader.gni
+++ b/build_overrides/vulkan_loader.gni
@@ -12,7 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-vulkan_headers_dir = "//third_party/vulkan-deps/vulkan-headers/src"
+import("//build_overrides/vulkan_common.gni")
 
-vulkan_gen_subdir = "vulkan_loader"
 vulkan_loader_shared = true