ARIA state/property aria-pressed is not exposed(Blink).

Source/modules/accessibility/AXNodeObject.cpp

 /*
 * Copyright (C) 2012, Google Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1.  Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 737 matching lines @@
 
 bool AXNodeObject::isPressed() const
 {
     if (!isButton())
         return false;
 
     Node* node = this->node();
     if (!node)
         return false;
 
-    // If this is an ARIA button, check the aria-pressed attribute rather than node()->active()
-    if (ariaRoleAttribute() == ButtonRole) {
-        if (equalIgnoringCase(getAttribute(aria_pressedAttr), "true"))
+    // ARIA button with aria-pressed not undefined, then check for aria-pressed attribute rather than node()->active()
+    if (ariaRoleAttribute() == ToggleButtonRole) {
+        if (equalIgnoringCase(getAttribute(aria_pressedAttr), "true")
+            || equalIgnoringCase(getAttribute(aria_pressedAttr), "mixed"))
             return true;
         return false;
     }
 
     return node->active();
 }
 
 bool AXNodeObject::isReadOnly() const
 {
     Node* node = this->node();
@@ skipping 1049 matching lines @@
     float range = maxValueForRange() - minValueForRange();
     float value = valueForRange();
 
     value += range * (percentChange / 100);
     setValue(String::number(value));
 
     axObjectCache()->postNotification(node(), AXObjectCacheImpl::AXValueChanged, true);
 }
 
 } // namespace blink