ash: Clarify OnWindowStateChanged parameter name

Be clear that the "type" is the old value.

BUG=None
TEST=compiles
R=oshima@chromium.org

Review URL: https://codereview.chromium.org/25952002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227010 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227010