athena: Fix a use-after-free case.
athena: Fix a use-after-free case.

|split_view_controller_| depends on |mru_window_tracker_|. So destroy them
in the correct order to avoid the use-after-free when destroying the window
manager.

BUG=none
TBR=mfomitchev@chromium.org, oshima@chromium.org

Review URL: https://codereview.chromium.org/471803002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@289464 0039d316-1c4b-4281-b951-d872f2087c98

ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,littlstar/chromium.src,bright-sparks/chromium-spacewalk,littlstar/chromium.src,ondra-novak/chromium.src,littlstar/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,littlstar/chromium.src,bright-sparks/chromium-spacewalk,littlstar/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,littlstar/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,littlstar/chromium.src,ondra-novak/chromium.src,littlstar/chromium.src