Treat inert nodes as aria-hidden instead of aria-disabled.

LayoutTests/accessibility/inert-node-is-hidden-expected.txt

Index: LayoutTests/accessibility/inert-node-is-hidden-expected.txt
diff --git a/LayoutTests/accessibility/inert-node-is-hidden-expected.txt b/LayoutTests/accessibility/inert-node-is-hidden-expected.txt
new file mode 100644
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+++ b/LayoutTests/accessibility/inert-node-is-hidden-expected.txt
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+This tests that inert elements are not in the AT tree.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS accessibilityController.accessibleElementById('input') is non-null.
+PASS treeContainsText(webArea, 'This content should not be exposed to AT when a modal dialog is open') is true
+PASS accessibilityController.accessibleElementById('input') is null
+PASS treeContainsText(webArea, 'This content should not be exposed to AT when a modal dialog is open') is false
+PASS accessibilityController.accessibleElementById('input') is non-null.
+PASS treeContainsText(webArea, 'This content should not be exposed to AT when a modal dialog is open') is true
+PASS successfullyParsed is true
+
+TEST COMPLETE
+This content should not be exposed to AT when a modal dialog is open.
+