LaCrOS: Define BUILDFLAG(IS_ASH).

In order to build lacros-chrome with target_os="chromeos", we will be
replacing existing defined(OS_CHROMEOS) with BUILDFLAG(IS_ASH) where
appropriate i.e. the parts only needed for ash-chrome.

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

Test: None
Bug: 1052397
Change-Id: Iee48f0a3cc37efbee1b7959c9340d1255e735a1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426071Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811054}

build/BUILD.gn

@@ -40,5 +40,6 @@ buildflag_header("chromeos_buildflags") {
   flags = [
     "IS_CHROMEOS_DEVICE=$is_chromeos_device",
     "IS_LACROS=$chromeos_is_browser_only",
+    "IS_ASH=$is_ash",
   ]
 }

build/config/chromeos/ui_mode.gni

@@ -7,6 +7,9 @@ declare_args() {
   # 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 'target_os = "chromeos" && ui_mode == "browser"'.
+  # replaced with is_lacros = chromeos_product == "browser".
   chromeos_is_browser_only = false
 }
+
+# TODO(crbug.com/1052397): Replace this with chromeos_product == "ash".
+is_ash = is_chromeos && !chromeos_is_browser_only