Bug 696179: properly cache null results for "alias" from the engineMetadataService so that we avoid hitting SQLite multiple times when entering text in the location bar, r=rflint

--HG--
extra : transplant_source : %17%3D%E7%D0%98%3B%D8Iv%F3UD%03%04%C2%E7%C0%3E%93%12

toolkit/components/search/nsSearchService.js

@@ -1005,8 +1005,9 @@ function Engine(aLocation, aSourceDataType, aIsReadOnly) {
 }
 
 Engine.prototype = {
-  // The engine's alias.
-  _alias: null,
+  // The engine's alias (can be null). Initialized to |undefined| to indicate
+  // not-initialized-from-engineMetadataService.
+  _alias: undefined,
   // The data describing the engine. Is either an array of bytes, for Sherlock
   // files, or an XML document element, for XML plugins.
   _data: null,
@@ -2201,7 +2202,7 @@ Engine.prototype = {
 
   // nsISearchEngine
   get alias() {
-    if (this._alias === null)
+    if (this._alias === undefined)
       this._alias = engineMetadataService.getAttr(this, "alias");
 
     return this._alias;