Set autocomplete attribute of search field on the flags page to off.

The issue here was the autocomplete feature, not autofill.
Autocomplete is disabled when the attribute is set to off.

Bug: 869516
Change-Id: Ib18953ed5e147cf6bab87b2c389ac75e83be13cd
Reviewed-on: https://chromium-review.googlesource.com/1249506
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594835}

components/flags_ui/resources/flags.html

@@ -23,7 +23,7 @@
   <div class="flex-container">
     <div class="flex">
       <div class="search-container">
-        <input type="text" id="search" placeholder="Search flags" tabindex="1" autocomplete="search">
+        <input type="text" id="search" placeholder="Search flags" tabindex="1" autocomplete="off">
         <button class="clear-search" title="Clear search" tabindex="2"></button>
       </div>
     </div>