[iOS/GN] Add a variable to configure the depth of "bundle_pool".

build/toolchain/mac/BUILD.gn

Index: build/toolchain/mac/BUILD.gn
diff --git a/build/toolchain/mac/BUILD.gn b/build/toolchain/mac/BUILD.gn
index bf5b31324a7f7eb35fe290c504be4822a4ab151b..231a546891d463367ebcdce18584116f5d5f92fa 100644
--- a/build/toolchain/mac/BUILD.gn
+++ b/build/toolchain/mac/BUILD.gn
@@ -33,12 +33,20 @@ if (!use_xcode_clang) {
       compiler_prefix + rebase_path("$clang_base_path/bin/", root_build_dir)
 }
 
-if (current_toolchain == default_toolchain) {
+declare_args() {
   # Reduce the number of tasks using the copy_bundle_data and compile_xcassets
   # tools as they can cause lots of I/O contention when invoking ninja with a
   # large number of parallel jobs (e.g. when using distributed build like goma).
+  bundle_pool_depth = -1
+}
+
+if (current_toolchain == default_toolchain) {
   pool("bundle_pool") {
-    depth = concurrent_links
+    if (bundle_pool_depth == -1) {
+      depth = concurrent_links
+    } else {
+      depth = bundle_pool_depth
+    }
   }
 }