Add isExpanded() API to WebAXObject.h to get aria-expanded attribute.
This patch also moves isExpanded() in AXObject to AXNodeObject like
other APIs to get attribute.

BUG=98982

Review URL: https://codereview.chromium.org/662313003

git-svn-id: svn://svn.chromium.org/blink/trunk@184058 bbb929c8-8fbe-4397-9dbb-9b2b20218538