Added basic accessibility to TreeView.

BUG=322729
TEST=manual with ChromeVox

Review URL: https://codereview.chromium.org/79543010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238047 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238047