android: Minor fix for Chrome official bundle builds.

The following CL contains a small fix required to ensure
that it is possible to build app bundle for official Chrome
builds, which require placeholders native libraries for
older Android releases.

BUG=820459
R=benmason@chromium.org,tiborg@chromium.org
TBR=agrieve@chromium.org

Change-Id: I1fa5999c808b11c248c854f6c5131c54996d9959
Reviewed-on: https://chromium-review.googlesource.com/1150173Reviewed-by: David Turner <digit@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: David Turner <digit@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577893}

build/config/android/rules.gni

@@ -2589,6 +2589,12 @@ if (enable_java_templates) {
                                ])
       }
     } else {
+      # placeholder native libraries never go into bundles, since they are
+      # only used for pre-L Android releases. Use the invoker variable to
+      # avoid GN errors when the target defines them.
+      assert(!defined(invoker.native_lib_placeholders) ||
+             invoker.native_lib_placeholders != [])
+
       _final_deps += [
                        ":$_merge_manifest_target",
                        ":$_build_config_target",