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The new breadcrumbs is positioned only on top of the file list/grid,
instead of on top of directory tree and file list/grid, this changes the
logical order of the tab sequence. Additionally, because the new
breadcrumb is a custom element with shadowRoot, using tabindex > 0 on
its sub-elements doesn't follow the desired order and having the
tabindex > 0 is not recommended by lighthouse [1].

This CL changes all Files app tabindex to 0 instead of greater than 0.
For previous design this doesn't change any tab order, for the new design
it has a different order and this is desired due to new breadcrumb
position, see bug for tab order details.

This CL updates the tab_index.js integration tests to have these 2
different tab orders.

Change helper function checkNextTabFocus() to dive in to shadowRoot to be
able to test the breadcrumbs sub-elements being focused.

[1] - https://developers.google.com/web/fundamentals/accessibility/focus/using-tabindex

Bug: 1059137
Test: browser_tests --gtest_filter="TabIndex/*"
Change-Id: Id39f2b1347235858b394229227e10288fe157b47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089192Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748996}