Fix ios_framework_bundle template when no public_headers defined

`_framework_headers_config` is needed by `_framework_public_config` for
the `framework_dirs` variable, even if no `public_headers` defined.

Introduced by https://crrev.com/c/2061194.

Change-Id: I82964f1266aa7c1764f33912b02b48ddf7677ba9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2098155
Commit-Queue: Bang Lee <banl@microsoft.com>
Auto-Submit: Bang Lee <banl@microsoft.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749190}

build/config/ios/rules.gni

@@ -1204,10 +1204,6 @@ template("ios_framework_bundle") {
         get_label_info("$_target_name($default_toolchain)", "target_gen_dir")
 
     _framework_headers_target = _target_name + "_framework_headers"
-    _framework_headers_config = _target_name + "_framework_headers_config"
-    config(_framework_headers_config) {
-      framework_dirs = [ _default_toolchain_root_out_dir ]
-    }
 
     _headers_map_config = _target_name + "_headers_map"
     _header_map_filename =
@@ -1218,6 +1214,11 @@ template("ios_framework_bundle") {
     }
   }
 
+  _framework_headers_config = _target_name + "_framework_headers_config"
+  config(_framework_headers_config) {
+    framework_dirs = [ _default_toolchain_root_out_dir ]
+  }
+
   _arch_shared_library_source = _target_name + "_arch_shared_library_sources"
   _arch_shared_library_target = _target_name + "_arch_shared_library"
   _lipo_shared_library_target = _target_name + "_shared_library"