LaCrOS: Replace is_chromeos with is_chromeos_ash

The change is mechanical replacing GN variable is_chromeos with
is_chromeos_ash. (For those cases please
refer to http://go/lacros-macros).

The patch is made in preparation to switching lacros build from
target_os=linux to target_os=chromeos. This will prevent lacros from
changing behaviour after the switch.

Bug: 1052397
Change-Id: Ib66985f4e40f01ec17be99c7ad1545403b243dab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594619
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837582}

diff --git a/testing/test.gni b/testing/test.gni
index 4f446e1..473b486 100644
--- a/testing/test.gni
+++ b/testing/test.gni

@@ -18,7 +18,7 @@
   import("//build/config/chromecast_build.gni")
   import("//build/config/fuchsia/generate_runner_scripts.gni")
   import("//build/config/fuchsia/package.gni")
-} else if (is_chromeos) {
+} else if (is_chromeos_ash) {
   import("//build/config/chromeos/rules.gni")
   import("//build/config/sanitizers/sanitizers.gni")
   import("//build/util/generate_wrapper.gni")
@@ -348,7 +348,7 @@
       data_deps += [ ":${_wrapper_output_name}" ]
       write_runtime_deps = _runtime_deps_file
     }
-  } else if (is_chromeos && cros_board != "") {
+  } else if (is_chromeos_ash && cros_board != "") {
     assert(!defined(invoker.use_xvfb) || !invoker.use_xvfb)
 
     # Building for a cros board (ie: not linux-chromeos).