Linux native notifications: Add initial unit test

chrome/test/BUILD.gn

Index: chrome/test/BUILD.gn
diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index 6419c2c6a8b7c690aca517c32dc6be2cd13f41c6..ca019697d52141039e0cd741d4cef65badd2d1c7 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -3651,6 +3651,10 @@ test("unit_tests") {
     ]
   }
 
+  if (is_desktop_linux && enable_native_notifications) {
+    sources += [ "../browser/notifications/notification_platform_bridge_linux_unittest.cc" ]
+  }
+
   if (is_linux || is_win) {
     sources += [ "../browser/webshare/share_service_impl_unittest.cc" ]
   }