Prevent mac_framework_bundle configs from getting reset

If the invoker sets "configs", the default configs would be overridden
by the forward_variables_from() call.  This fixes linker errors for
certain shared libraries on Mac Asan when rolling in the change that
switches Mac to build with our in-tree libc++.

BUG=chromium:924121
R=yvesg
CC=thakis

Change-Id: If9db41b724f891034086c64dc7ba38a6406aef92
Reviewed-on: https://webrtc-review.googlesource.com/c/119286
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Yves Gerey <yvesg@google.com>
Commit-Queue: Tom Anderson <thomasanderson@google.com>
Cr-Commit-Position: refs/heads/master@{#26377}

webrtc.gni

@@ -717,7 +717,10 @@ if (is_mac) {
     modulemap_path = "$target_gen_dir/Modules/module.modulemap"
 
     mac_framework_bundle(target_name) {
-      forward_variables_from(invoker, "*", [])
+      forward_variables_from(invoker, "*", [ "configs" ])
+      if (defined(invoker.configs)) {
+        configs += invoker.configs
+      }
 
       framework_version = "A"
       framework_contents = [