Bug 233099

From https://html.spec.whatwg.org/multipage/interaction.html#inert, > A Document document is blocked by a modal dialog subject if subject is > the topmost dialog element in document's top layer. While document is > so blocked, every node that is connected to document, with the > exception of the subject element and its shadow-including descendants, > must be marked inert. RenderStyle::effectiveInert() already matches this definition, Node::deprecatedIsInert() does not. Main reason for the removed check was to prevent the whole document from being inert to hit-testing, but with the RenderStyle approach, we instead override effectiveInert to false for the modal dialog. Removing this check for focus is absolutely fine however, since focusability isn't inherited. Tests added by this Chromium WPT: https://github.com/web-platform-tests/wpt/commit/0457111e7109ec3d9e575aa421b96d8c36ce2ae8