[inert] Let modal dialogs escape inertness from ancestors

If an ancestor of a modal <dialog> has the 'inert' attribute, the dialog
and its subtree will no longer be inert.

However, if the 'inert' attribute is in the <dialog> itself, then the
subtree will still be inert.

This aligns Blink with Gecko and WebKit. The spec conversation about
this topic is in whatwg/html#7808

Bug: 1323680

TEST=external/wpt/inert/inert-with-modal-dialog-001.html
TEST=StyleResolverTest.IsInertWithAttributeAndDialog

Change-Id: Id2752ce3d1c8985b68b1d700710c3ff3eb100714

inert/inert-with-modal-dialog-001.html

@@ -3,7 +3,7 @@
 <title>Interaction of 'inert' attribute with modal dialog</title>
 <link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
 <link rel="help" href="https://html.spec.whatwg.org/multipage/interaction.html#inert">
-<meta name="assert" content="Checks that being part of a modal dialog does not protect a node from being marked inert by an 'inert' attribute.">
+<meta name="assert" content="Checks that a modal dialog escapes inertness from ancestors.">
 <div id="log"></div>
 <div id="wrapper">
   wrapper
@@ -51,6 +51,6 @@
 test(function() {
   wrapper.inert = true;
   this.add_cleanup(() => { wrapper.inert = false; });
-  checkSelection("");
-}, "If an ancestor of the dialog has the 'inert' attribute, everything becomes inert");
+  checkSelection("dialog child");
+}, "If an ancestor of the dialog has the 'inert' attribute, the dialog escapes inertness");
 </script>