[lacros] Run Lacros test targets with ash_chrome if needed

This CL makes two changes:
1. Make use_xvfb imply use_ash_chrome.
2. Mark test targets to use xvfb and ash_chrome if needed.

Bug: 1104318
Change-Id: I7128b31b83e2389e92e5507093396aaf537dfdf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2324765
Reviewed-by: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792730}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index fd917270..53e7715d 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -306,6 +306,10 @@
 }
 
 test("aura_unittests") {
+  if ((is_linux && !is_chromeos) || chromeos_is_browser_only) {
+    use_xvfb = true
+  }
+
   sources = [
     "../compositor_extra/shadow_unittest.cc",
     "gestures/gesture_recognizer_unittest.cc",