Indicate sw.js and background.js should be kept in sync

Change-Id: Ic8c3917f252a4a3817b510838ecac6ec37f0810c
diff --git a/src/sw.js b/src/sw.js
index c90c5a6..c214068 100644
--- a/src/sw.js
+++ b/src/sw.js
@@ -1,3 +1,5 @@
+// IMPORTANT: keep this file in sync with background.js
+
 importScripts('common/common.js')
 
 // When the extension gets updated, set new options to their default value.