Add note about overriding destroy().
Add note about overriding destroy().

This is, usually, the wrong override point. Make a note in the class to
point people at willBeDestroyed().

BUG=490314

Review URL: https://codereview.chromium.org/1151003002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@195727 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk