ChromeOS: Expand build flag comment.

Explain that chromeos_is_browser_only is set on ChromeOS when using Wayland.

Change-Id: I7649ef51b8c7759f4dfed0021bf32bcc2c8f6886
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2324663Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792502}

build/config/chromeos/ui_mode.gni

@@ -4,7 +4,8 @@
 
 declare_args() {
   # This controls UI configuration for Chrome.
-  # If this flag is set, we assume Chrome runs on Chrome OS devices.
+  # 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"'.
   chromeos_is_browser_only = false