Add exclude_imports flag to proto_library

|exclude_imports| flag is set in some of the perfetto's proto_library
targets to indicate that we don't need to generate the proto-descriptor
for the protos included in those `x.proto` files. In this CL we use that
flag to conditionally pass `--include_imports` argument to protoc.

This is similar to the CL (https://crrev.com/c/2632759)

Bug: b:236945541
Change-Id: I0689003978096798d1e966ec8485cd6af7237804
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3721616
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Mohit Saini <mohitms@google.com>
Cr-Commit-Position: refs/heads/main@{#81356}

gni/proto_library.gni

@@ -76,6 +76,17 @@ template("proto_library") {
   out_dir = "$root_gen_dir/" + proto_out_dir
   rel_out_dir = rebase_path(out_dir, root_build_dir)
 
+  # exclude_imports is only used for generating the descriptor. Therefore, the
+  # check needs to be here to avoid complaints from GN about the unused
+  # variable.
+  if (generate_descriptor != "") {
+    if (defined(invoker.exclude_imports)) {
+      exclude_imports = invoker.exclude_imports
+    } else {
+      exclude_imports = false
+    }
+  }
+
   # Prevent unused errors when generating descriptor only.
   if (generate_descriptor != "") {
     not_needed([ "rel_out_dir" ])
@@ -163,8 +174,10 @@ template("proto_library") {
     }
     if (generate_descriptor != "") {
       depfile = "$out_dir/$generate_descriptor.d"
+      if (!exclude_imports) {
+        args += [ "--include_imports" ]
+      }
       args += [
-        "--include_imports",
         "--descriptor_set_out",
         rebase_path("$out_dir/$generate_descriptor", root_build_dir),
         "--dependency_out",