ozone: Make sure the web-contents and root-window are cleaned up correctly.

Also make sure a WindowTreeClient is installed properly for the client (after r230501).
This fixes running most of the content_browsertests for ozone.

BUG=none
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/38583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230739 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230739