Reland r256396 Add views implementation for AXTreeSource.

BUG=318758
TBR=dtseng@chromium.org

Review URL: https://codereview.chromium.org/196603003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256563 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256563