Bug 1566698 [wpt PR 17869] - Create an attribute and property for toa…

…st closebutton and attach close functionality, a=testonly

Automatic update from web-platform-tests
Create an attribute and property for toast closebutton and attach close functionality

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
jackbsteinberg/std-toast#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/+/1699269
Reviewed-by: Fergal Daly <fergalchromium.org>
Reviewed-by: Kent Tamura <tkentchromium.org>
Commit-Queue: Jack Steinberg <jacksteinbergchromium.org>
Cr-Commit-Position: refs/heads/master{#678530}

--

wpt-commits: bd0b2bff5361391e8c25949aa40f3167dd82d572
wpt-pr: 17869

UltraBlame original commit: 8930d78b9176fc46cc05bc4f0f3f1465128d1da1

testing/web-platform/tests/std-toast/attributes.html

@@ -929,6 +929,9 @@
 '
 action
 '
+'
+closeButton
+'
 ]
 ;
 assert_array_equals