M131: [A11y] Handle when document is marked dirty late in the process

This condition actually occurs only on Mac, because it is the only
platform that needs to use MarkDocumentDirty() when aria-modal dialogs
are created or destroyed. There is no real additional cost to keep the
code for all platforms.

Will do test as a follow-up. Repro is to load pepboys.com and
continually click on "Your store" on the right side of the toolbar.

(cherry picked from commit f2eb83b5)

Fixed: 372508699,375669433
Change-Id: I524479e035d27e6118a53fdccef77fc9ad5d7dcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5979112


Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1376030}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5981914
Cr-Commit-Position: refs/branch-heads/6778@{#1719}
Cr-Branched-From: b21671ca-refs/heads/main@{#1368529}