Move root_window_host* in aura to window_tree_host*.

R=sky@chromium.org
TBR=sky@chromium.org
BUG=

Review URL: https://codereview.chromium.org/160573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251379 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251379