Make TreeOutline in Changes not focusable until there is content
Make TreeOutline in Changes not focusable until there is content

When the empty TreeOutline in Changes panel gets focus via keyboard,
the outline is shown without any content. This change disable focus
until there is some content in the aforementioned TreeOutline.

Bug: 1048986
Change-Id: I9aa9c24a0197a9fd23a862cc803ce785196a6a2f
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2132236
Commit-Queue: Peter Marshall <6d0eecbeb7fb9d172f49b4b32481d2466fd89429@chromium.org>
Reviewed-by: Jan Scheffler <8abd9e3d93041d096604cb2a1b3070dd7075ba7b@chromium.org>

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