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This change sets the groundwork for the toast closebutton, by
adding an attribute to set the closebutton content,
creating a closeButton property to reflect that attribute,
and adding an event listener to close the toast when the closebutton is pressed.

This change implements the close button API described in
(https://github.com/jackbsteinberg/std-toast/blob/eec7728f7082a897d777181ac07b0448062ffca5/README.md),
and is the subject of debate on issue #48 on the explainer here
https://github.com/jackbsteinberg/std-toast/issues/48.

Future CLs will add the closeButton option for showToast
(https://github.com/jackbsteinberg/std-toast/tree/master#showtoastmessage-options)
Link: https://chromium-review.googlesource.com/c/chromium/src/+/1706453

Bug: 972945
Change-Id: I5d6c3055791a9f93ef414e575e128c61e2a944ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1699269Reviewed-by: Fergal Daly <fergal@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Jack Steinberg <jacksteinberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678530}