Make the js tree view accessible with ChromeVox

Previously, the treeview did not have annotations for accessibility (which is used by ChromeVox), so all the items in the treeview were spoken at one time when the treeview got the focus.

This patch adds an attribute which indicates a focused item, so only the selected item is spoken when the treeview is focused. And when the selected item is changed, new selected item is spoken.

BUG=295899
TEST=manually tested

Review URL: https://codereview.chromium.org/143823018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247186 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247186