Oilpan: tidy up transition type usage after r178508.

The Image object is not on the heap, adjust transition type usage to
reflect that. Not doing so lead to

  virtual/threaded/compositing/webgl/webgl-copy-image.html

failing in Oilpan builds --

 http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=virtual%2Fthreaded%2Fcompositing%2Fwebgl%2Fwebgl-copy-image.html

TBR=haraken@chromium.org,oilpan-reviews@chromium.org
BUG=392765
NOTRY=true

Review URL: https://codereview.chromium.org/401013003

git-svn-id: svn://svn.chromium.org/blink/trunk@178514 bbb929c8-8fbe-4397-9dbb-9b2b20218538