make bookmarks stop disappearing when a user unselects a microsummary (i.e. goes back to the static page title)

patch by Simon B��nzli
bug=342463
r=myk

browser/components/microsummaries/src/nsMicrosummaryService.js

@@ -655,6 +655,11 @@ MicrosummaryService.prototype = {
                         this._resource(FIELD_RDF_TYPE),
                         this._resource(VALUE_MICSUM_BOOKMARK),
                         this._resource(VALUE_NORMAL_BOOKMARK));
+
+    // If we're clearing a field that could affect this bookmark's label,
+    // then force all bookmark trees to rebuild from scratch.
+    if (fieldName == FIELD_MICSUM_GEN_URI || fieldName == FIELD_GENERATED_TITLE)
+      this._forceBookmarkTreesRebuild();
   },
 
   _hasField: function MSS__hasField(bookmarkID, fieldName) {