WebUI: Disable autocomplete on cr-inputs

Autocomplete seems get activated if the user has submitted a form with
the same ID or name on another website. Given that WebUI does not
specify names or use cr-inputs within form elements, it seems safe to
assume that autocompletion on cr-inputs would in general be not
helpful as they will be autocompleting off values submitted on any
website that has an input with an ID of 'input'.

Bug: 1134578
Change-Id: I066bdd74ac60274346113b03489a98797c2f3d05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453589Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815414}

ui/webui/resources/cr_elements/cr_input/cr_input.html

@@ -137,7 +137,8 @@
               aria-invalid$="[[getAriaInvalid_(invalid)]]"
               max="[[max]]" min="[[min]]" on-focus="onInputFocus_"
               on-blur="onInputBlur_" on-change="onInputChange_"
-              on-keydown="onInputKeydown_" part="input">
+              on-keydown="onInputKeydown_" part="input"
+              autocomplete="off">
           <slot name="inline-suffix"></slot>
         </div>
         <div id="underline"></div>