wm: fix close window animation BUG=chromium-os:5329 TEST=try animation with mutliple windows
wm: fix close window animation
BUG=chromium-os:5329
TEST=try animation with mutliple windows

Review URL: http://codereview.chromium.org/3124020

joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager