Fix memory leak in TransientWindowStackingClientTest caused by r242126.

http://src.chromium.org/viewvc/chrome/trunk/src/ui/aura/client/window_stacking_client.cc?r1=241867&r2=242126
changed SetWindowStackingClient() to no longer delete the old client, but it
didn't update these tests which relied on that behavior. Explicitly delete this
object in the test.

BUG=330382
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/120503004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242330 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242330