[directorytree] The splitter button should have tabindex -1

The splitter button has no keyboard interaction so no need for it to
receive tab focus. Tab focus from directorytree => to breadcrumb, is
the behavior we want.

Bug: 1002790, 992819
Change-Id: If41272a1edd903be36b858aed211083f2c6ebdd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087204Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747125}

ui/file_manager/file_manager/main.html

@@ -499,7 +499,7 @@
         </div>
       </div>
       <div class="splitter" id="navigation-list-splitter">
-        <cr-button class="splitter-button">
+        <cr-button class="splitter-button" tabindex="-1">
           <div class="icon"></div>
         </cr-button>
       </div>