Remove PanelLayoutManager's LauncherIconObserver when destroyed.
Remove PanelLayoutManager's LauncherIconObserver when destroyed.

Because we never set launcher_, we never called RemoveObserver.

BUG=none
TEST=none


Review URL: http://codereview.chromium.org/10209019

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@134235 0039d316-1c4b-4281-b951-d872f2087c98

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