Rename root_window_host.h to window_tree_host.h

Also renames root_window_host_delegate.h to window_tree_host_delegate.h

Doing this in a change before updating the contents of the files to make the code review tool happy.

TBR=sky@chromium.org
http://crbug.com/308846

Review URL: https://codereview.chromium.org/68893010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234767 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234767