libdot: prefs: fix handling of null defaults

In JS, null is an object, so the "not simple primitive type" test
ends up considering all null settings as "not the default".  Add
an explicit test for the null object to catch that.

Change-Id: Ia9a011d1eb1e741cec584f023b2e75d20ba019bf
Reviewed-on: https://chromium-review.googlesource.com/c/1352191
Tested-by: Mike Frysinger <[email protected]>
Reviewed-by: Vitaliy Shipitsyn <[email protected]>

Author: vapier

libdot/js/lib_preference_manager.js

@@ -604,12 +604,22 @@ lib.PreferenceManager.prototype.resetAll = function() {
  * @param {*} b A value to compare.
  */
 lib.PreferenceManager.prototype.diff = function(a, b) {
-  // If the types are different, or the type is not a simple primitive one.
-  if ((typeof a) !== (typeof b) ||
-      !(/^(undefined|boolean|number|string)$/.test(typeof a))) {
+  // If the types are different.
+  if ((typeof a) !== (typeof b)) {
     return true;
   }
 
+  // Or if the type is not a simple primitive one.
+  if (!(/^(undefined|boolean|number|string)$/.test(typeof a))) {
+    // Special case the null object.
+    if (a === null && b === null) {
+      return false;
+    } else {
+      return true;
+    }
+  }
+
+  // Do a normal compare for primitive types.
   return a !== b;
 };