Bug 1435588 - (part 2) Request records oldest first with the bookmarks buffer r=kitcambridge

This also adds a check that the sort order is valid to record.js,
since I accidentally spelt it "oldest", which didn't work at all.

MozReview-Commit-ID: 2edyyjcwGqx

--HG--
extra : rebase_source : 051378c4f79f703563a1e14a32a28956f9dcf12b

services/sync/modules/engines/bookmarks.js

@@ -693,6 +693,10 @@ BufferedBookmarksEngine.prototype = {
   // non-buffered engine is enabled.
   overrideTelemetryName: "bookmarks-buffered",
 
+  // Needed to ensure we don't miss items when resuming a sync that failed or
+  // aborted early.
+  _defaultSort: "oldest",
+
   async getLastSync() {
     let mirror = await this._store.ensureOpenMirror();
     return mirror.getCollectionHighWaterMark();

services/sync/modules/record.js

@@ -685,6 +685,10 @@ Collection.prototype = {
   // index
   get sort() { return this._sort; },
   set sort(value) {
+    if (value && value != "oldest" && value != "newest" && value != "index") {
+      throw new TypeError(
+        `Illegal value for sort: "${value}" (should be "oldest", "newest", or "index").`);
+    }
     this._sort = value;
     this._rebuildURL();
   },