Implement new focusing steps for modal <dialog>.

LayoutTests/fast/dom/HTMLDialogElement/show-modal-focusing-steps-expected.txt

Index: LayoutTests/fast/dom/HTMLDialogElement/show-modal-focusing-steps-expected.txt
diff --git a/LayoutTests/fast/dom/HTMLDialogElement/show-modal-focusing-steps-expected.txt b/LayoutTests/fast/dom/HTMLDialogElement/show-modal-focusing-steps-expected.txt
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index 0000000000000000000000000000000000000000..6504e8299e58a41f3c2448262dcbc36a9e2b8ea0
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+Tests focus when a modal dialog is opened.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS successfullyParsed is true
+
+TEST COMPLETE
+PASS document.activeElement is outerButton
+Test that focus goes to body if the dialog has no focusable elements, including itself
+PASS document.activeElement is document.body
+Test that an autofocus element in the dialog gets focus.
+PASS document.activeElement is autofocusButton
+... or else first focusable element in the dialog gets focus.
+PASS document.activeElement is firstButton
+... or else the dialog itself gets focus.
+PASS document.activeElement is dialog
+PASS successfullyParsed is true
+
+TEST COMPLETE
+