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This CL fixes the keyboard navigation when using screen reader/
Chromevox.

The main fix is to force focus on the cr.ui.Tree/DirectoryTree element
when navigating with Chromevox keyboard shortcut "Search+right arrow",
before the focus was staying on "body" element so Tree couldn't handle
the keyboard events to expand/collapse directories.

It's a series of small fixes.

1. Fix text read by Chromevox when navigating on directories, it was
reading all sub-directories names, instead of only the current focused
directory's name, fixed by adding |id| to each entries' label element,
to make existing |aria-labelledby| attribute work for screen reader.
This was broken because we overwrite the |innerHTML| from
|cr.ui.TreeItem|, losing the |id|.

2. Listen to |click| event on DirectoryTree and forces the focus to it
when the focus/activeElement is the "body" element. This happens when
Chromevox issues the |click| event when user press "Search+right arrow".

3. Replace |cr.ui.Tree|'s |role| attribute from "group" to "navigation",
which is a "landmark" type which is recommended [1]. This required to
change |cr.ui.Tree| to not overwrite existing |role| attribute.


[1] - https://developers.google.com/web/fundamentals/accessibility/how-to-review#take_advantage_of_headings_and_landmarks

Bug: 755278
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: If33be610e1d28f0a69205ee148783328a367fe80
Reviewed-on: https://chromium-review.googlesource.com/985772
Commit-Queue: Luciano Pacheco (SYD) <lucmult@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Sasha Morrissey <sashab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553917}