Set tabindex of search box -1 not to be focused by default.

BUG=408949
TEST=manually

Review URL: https://codereview.chromium.org/529693002

Cr-Commit-Position: refs/heads/master@{#292859}

ui/file_manager/file_manager/main.html

@@ -312,7 +312,7 @@
           <div class="topbutton-bar">
             <button class="menubutton icon search-button" id="search-button" tabindex="7"></button>
             <div id="search-box">
-              <input type="search" tabindex="99"
+              <input type="search" tabindex="-1"
                      i18n-values="aria-label:SEARCH_TEXT_LABEL;placeholder:SEARCH_TEXT_LABEL">
               <button class="clear"></button>
             </div>