Lacros: Replace gn var is_chromeos with is_chromeos_ash.

This change is a mechanical change replacing is_chromeos with
is_chromeos_ash.

This is in preparation to switching lacros from target_os="linux" to
target_os="chromeos".

More detail on http://go/lacros-macros.

Bug: 1052397
Change-Id: I714585aea9ccd0973551c89fc535331b39ce713d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628710
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: Yuta Hijikata <ythjkt@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843657}

chromeos/services/assistant/public/cpp/migration/BUILD.gn

 # Copyright 2020 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
+import("//build/config/chromeos/ui_mode.gni")
 import("//chromeos/assistant/assistant.gni")
 
-assert(is_chromeos)
+assert(is_chromeos_ash)
 assert(enable_cros_libassistant)
 
 component("migration") {