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For images without any alternative text, there is no point to add it
into the accessibility tree as it will only be recorded as an image.
We could safely ignore such image as it doesn't convey any extra
information.

One such case is the Chrome logo icon on HaTS bubble. We don't need to
link it into accessibility tree.

BUG=979530

Change-Id: I55d1dceb33f5d55241470a5cbd8ff66be9dca877
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1783672Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693802}