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This largely seems to create a bunch of noise for screenreaders whenever
the preview is reloaded, unless another element is explicitly focused.
The message is also generally unhelpful, e.g. Chrome Vox reads out
the name of the underlying WebContent, iframe, and the names of all the
plugin buttons, which seems unlikely to be useful.

Bug: 987370
Change-Id: I97b7d25b859e50bd51b82ffa6760571ae8319a73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2573881Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833758}