Fix type of window parameter in onShown event

BUG=none
TEST=observe content of devtools.panels.html
R=cduvall@chromium.org

Review URL: https://codereview.chromium.org/12084019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257074 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 257074