Set tabindex on the files app search box clear button.

Bug: 849618
Change-Id: I768ea72228e4fb2c322eeb0e04d00075c677a826
Reviewed-on: https://chromium-review.googlesource.com/c/1343874Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609641}

ui/file_manager/file_manager/main.html

@@ -362,7 +362,7 @@
         <div id="search-box">
           <cr-input type="search" tabindex="14" hidden
               i18n-values="aria-label:SEARCH_TEXT_LABEL;placeholder:SEARCH_TEXT_LABEL">
-            <span class="clear" slot="suffix"></span>
+            <span class="clear" slot="suffix" tabindex="14"></span>
           </cr-input>
         </div>
         <button id="refresh-button" class="icon-button menu-button" tabindex="15" hidden