Bug 1489440 - Disable autocomplete in the XUL autocomplete bindings. r=dao

With the patch above we do find the input element, and try to autocomplete it
normally, which confuses some tests.

Differential Revision: https://phabricator.services.mozilla.com/D17143

--HG--
extra : moz-landing-system : lando

toolkit/content/widgets/autocomplete.xml

@@ -18,6 +18,7 @@
         <children/>
         <html:input anonid="input" class="textbox-input"
                     allowevents="true"
+                    autocomplete="off"
                     xbl:inherits="value,type=inputtype,maxlength,disabled,size,readonly,placeholder,tabindex,accesskey,mozactionhint"/>
       </xul:moz-input-box>
       <children includes="hbox"/>