Added debug logging to the Image create/load/GC cycle/

Logging will help debug intermittent failures to dispatch load events.

BUG=407976

Review URL: https://codereview.chromium.org/535663003

git-svn-id: svn://svn.chromium.org/blink/trunk@181333 bbb929c8-8fbe-4397-9dbb-9b2b20218538