[lacros] Fix ui_base_unittests linking failure on lacros

The breakage was introduced in:
https://chromium-review.googlesource.com/c/chromium/src/+/2315863

lacros-chrome is built on Linux, but doesn't use x11, so this CL fixes
the assumption.

Bug: 1110072
Change-Id: I85293dd604a390d8cc97e5f03f637db40d1d5812
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2321237Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792423}

ui/base/clipboard/BUILD.gn

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("///ui/ozone/ozone.gni")
+import("//build/config/chromeos/ui_mode.gni")
 import("//build/config/jumbo.gni")
 import("//build/config/ui.gni")
 
@@ -231,7 +232,7 @@ source_set("clipboard_test") {
     ]
   }
 
-  if (use_aura && !use_x11 && !is_win) {
+  if (use_aura && !chromeos_is_browser_only && !use_x11 && !is_win) {
     sources += [ "clipboard_data_unittest.cc" ]
   }