Bug 1400595 - Allow Preferences search engine table scrollable only i…

…f its items are overflow r=jaws

MozReview-Commit-ID: DVqCYNkh4x8

--HG--
extra : rebase_source : 1fc221b69344e2d6d38b0564983718cdb7c7ef41

browser/components/preferences/in-content/search.xul

@@ -72,7 +72,7 @@
       <description>&chooseWhichOneToDisplay2.label;</description>
 
       <tree id="engineList" flex="1" rows="8" hidecolumnpicker="true" editable="true"
-            seltype="single">
+            seltype="single" allowunderflowscroll="true">
         <treechildren id="engineChildren" flex="1"/>
         <treecols>
           <treecol id="engineShown" type="checkbox" editable="true" sortable="false"/>