Rename RootWindowHost* to WindowTreeHost*

TBR=sky@chromium.org
http://crbug.com/308843

Review URL: https://codereview.chromium.org/76583003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236048 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236048