BUILD.gn: Delete stale generated files in Dawn's gen dir.

This normalizes even more the directory structure of generated files in
Dawn and removes stale autogenerated files that could be included
wrongly using a GN action.

See comment on top of dawn_generator.gni in this commit for more
context.

BUG=dawn:22

Change-Id: I8ec038f949c048431b2b643af4462f98c4ae610b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/11361
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/generator/generator_lib.gni b/generator/generator_lib.gni
index 10c560d..36ac72f 100644
--- a/generator/generator_lib.gni
+++ b/generator/generator_lib.gni
@@ -24,6 +24,8 @@
 #   outputs: List of expected outputs, generation will fail if there is a
 #     mistmatch.
 #
+#   deps: additional deps for the code generation targets.
+#
 #   generator_lib_dir: directory where generator_lib.py is located.
 #
 #   custom_gen_dir: Optional custom target gen dir. Defaults to $target_gen_dir
@@ -138,6 +140,9 @@
     ]
     depfile = _json_tarball_depfile
     args = _generator_args
+    if (defined(invoker.deps)) {
+      deps = invoker.deps
+    }
   }
 
   # Extract the JSON tarball into the gen_dir