Lacros: Prepare gn file for lacros target_os switch.

This is in preparation to switching lacros-chrome from target_os="linux"
to target_os="chromeos". The change is made so that the behaviour of
lacros-chrome is consistent.

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

Bug: chromium:1052397
Change-Id: Icfc3a9418fb83c9d91e0ecdce8cab9db48cc1ea7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636906
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Yuta Hijikata <ythjkt@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844788}

chrome/browser/resources/pdf/BUILD.gn

@@ -113,7 +113,7 @@ generate_grd("build_grd") {
     "index.html",
     "main.js",
   ]
-  if (is_chromeos) {
+  if (is_chromeos_ash) {
     input_files += [
       "ink/index.html",
       "ink/ink_api.js",