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Now we have the QuickView <webview> infra to detect the 'loaded' state
of the <webview> content, add a <file-safe-media> audio test, and also
check the <webview> renders its content on a transparent black body.

This test exposed an issue in Blink node code. When the modal <dialog>
is calling showModal(), its flat tree distribution can be out-of-date,
causing a DCHECK crash when testing if the Quick View <dialog> element
contains sibling <iframe id="command-dispatcher"> [1].

Fix that: FlatTreeTraversal::ContainsIncludingPseudoElement() requires
that its node arguments have up-to-date flat tree distribution. Ensure
the <dialog>'s flat tree distribution is up-of-date _before_ trying to
propagate the "inert" bit into sub-frames.

Add crash test: html/dialog/showmodal-shadow-sibling-frame-crash.html

[1] Known crash reports already exist: issue 789094 issue 804047.

test: browser_tests --gtest_filter="QuickView*openQuickViewAudio"
Bug: 891150, 789094, 804047
Change-Id: I714585272fc775c157f6d0bd97143af27bf2b961
Reviewed-on: https://chromium-review.googlesource.com/c/1264138
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599125}