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Messaging in cr-toasts were not being read out consistently, especially
in cases where the toast text stays the same for multiple actions (for
example, "Bookmark removed." for multiple different bookmarks).

This CL updates the cr-toast's show method to remove the role="alert"
attribute and re-add it to better ensure screen readers recognize to
read out the cr-toast's contents, and fixes one use of cr-toast to use
the show method instead of data-binding a property as the open
attribute.

Using a method is better than using an attribute because each call to
the method is a signal to read out the cr-toast, while using an
attribute will not trigger for any toasts that are shown while a toast
is already visible. For example, setting |open| to true while it is
already set to true will not trigger any JS, but calling |show()| is
guaranteed to always trigger the method's contents.

The plan is to replace all uses of the open attribute to the show method
for better consistency, and to eventually change the open property on
cr-toast to read-only.

Previous params for show were not being used.

Fixed: 1055842
Change-Id: I6c545814a4fc4b465b35708abbfa746c4acbb69a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2477663
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818616}