Add a window parameter to WindowDelegate's OnWindowDestroying/Destroyed methods.

This aligns this aspect of WindowDelegate with WindowObserver, that I hope to replace this part of WD with in the future. It also allows me to clean up a little bit of the DWTH/DNWA interaction (subsequent CL).

BUG=none
R=sky@chromium.org

Review URL: https://codereview.chromium.org/183143006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253983 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253983