LaCrOS: Define is_lacros as alias for chromeos_is_browser_only.

Define is_lacros GN variable as an alias for chromeos_is_browser_only.

In the future we plan to move to is_lacros and is_ash both derived from
GN variable chromeos_product. However we want to introduce the is_lacros
variable before that so that developers can start using it now.

For more detail please read the design doc go/lacros-build-config.

Test: None
Bug: 1052397
Change-Id: I33d6555ccd1ec4d7a08a659eab25c3709887e63b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444876
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Auto-Submit: Yuta Hijikata <ythjkt@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813331}

build/config/chromeos/ui_mode.gni

@@ -3,13 +3,21 @@
 # found in the LICENSE file.
 
 declare_args() {
+  # Deprecated, use is_lacros.
+  #
   # This controls UI configuration for Chrome.
   # If this flag is set, we assume Chrome runs on Chrome OS devices, using
   # Wayland (instead of X11).
-  # TODO(crbug.com/1052397): After the clean up, this should be
-  # replaced with is_lacros = chromeos_product == "browser".
+  #
+  # TODO(crbug.com/1052397):
+  # Define chromeos_product instead, which takes either "browser" or "ash".
+  # Re-define the following variables as:
+  # is_lacros = chromeos_product == "browser"
+  # is_ash = chromeos_product == "ash"
   chromeos_is_browser_only = false
 }
 
-# TODO(crbug.com/1052397): Replace this with chromeos_product == "ash".
 is_ash = is_chromeos && !chromeos_is_browser_only
+
+# Temporarily define is_lacros as an alias.
+is_lacros = chromeos_is_browser_only