Update the accessibility tree when a modal dialog is opened/closed.

Source/core/html/HTMLDialogElement.cpp

Index: Source/core/html/HTMLDialogElement.cpp
diff --git a/Source/core/html/HTMLDialogElement.cpp b/Source/core/html/HTMLDialogElement.cpp
index b264572319d7a1f09e51040bdb1d235e4718543a..f93b4129afb7485f13ec29fd4f495f2493200f00 100644
--- a/Source/core/html/HTMLDialogElement.cpp
+++ b/Source/core/html/HTMLDialogElement.cpp
@@ -27,6 +27,7 @@
 #include "core/html/HTMLDialogElement.h"
 
 #include "bindings/v8/ExceptionState.h"
+#include "core/accessibility/AXObjectCache.h"
 #include "core/dom/ExceptionCode.h"
 #include "core/dom/NodeTraversal.h"
 #include "core/html/HTMLFormControlElement.h"
@@ -57,6 +58,12 @@ static void runAutofocus(HTMLDialogElement* dialog)
     }
 }
 
+static void inertSubtreesChanged(Document& document)
+{
+    if (AXObjectCache* cache = document.existingAXObjectCache())
+        cache->handleInertSubtreesChanged();
+}
+
 HTMLDialogElement::HTMLDialogElement(const QualifiedName& tagName, Document& document)
     : HTMLElement(tagName, document)
     , m_centeringMode(Uninitialized)
@@ -86,7 +93,11 @@ void HTMLDialogElement::closeDialog(const String& returnValue)
     if (!fastHasAttribute(openAttr))
         return;
     setBooleanAttribute(openAttr, false);
+
+    HTMLDialogElement* activeModalDialog = document().activeModalDialog();
     document().removeFromTopLayer(this);
+    if (activeModalDialog == this)
+        inertSubtreesChanged(document());
 
     if (!returnValue.isNull())
         m_returnValue = returnValue;
@@ -117,6 +128,7 @@ void HTMLDialogElement::showModal(ExceptionState& es)
         return;
     }
     document().addToTopLayer(this);
+    inertSubtreesChanged(document());
     setBooleanAttribute(openAttr, true);
 
     runAutofocus(this);