Simplify proto descriptor rules

Remove a couple of unnecessary path mangling steps and
fix ftrace.descriptor generation to be in the correct place.

Bug: 139319295
Change-Id: I7165ee70f8051ef50c511e17c3dbf47986ec1f1a
diff --git a/gn/standalone/proto_library.gni b/gn/standalone/proto_library.gni
index f79ae07..0180b2b 100644
--- a/gn/standalone/proto_library.gni
+++ b/gn/standalone/proto_library.gni
@@ -78,7 +78,7 @@
   protogens = []
 
   if (generate_descriptor != "") {
-    protogens += [ "$root_gen_dir/" + generate_descriptor ]
+    protogens += [ "$target_gen_dir/" + generate_descriptor ]
   }
 
   foreach(proto, protos) {
@@ -144,14 +144,13 @@
       ]
     }
     if (generate_descriptor != "") {
-      depfile = "$target_gen_dir/" +
-                rebase_path("$generate_descriptor.d", root_gen_dir)
+      depfile = "$target_gen_dir/$generate_descriptor.d"
       args += [
         "--include_imports",
         "--descriptor_set_out",
-        rebase_path("$root_gen_dir/" + generate_descriptor, root_build_dir),
+        rebase_path("$target_gen_dir/$generate_descriptor", root_build_dir),
         "--dependency_out",
-        rebase_path("$root_gen_dir/$generate_descriptor.d", root_build_dir),
+        rebase_path(depfile, root_build_dir),
       ]
     }