Oilpan: tidy up transition type usage after r178508.

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178514

[sof, 2014-07-19 06:58:57]

Please take a look.

[sof, 2014-07-19 07:02:51]

The use of "regressing" in the description was wrong (it is a new test that
r178508 nicely added); updated the description to reflect that.

[sof, 2014-07-19 07:09:00]

The CQ bit was checked by sigbjornf@opera.com

[commit-bot: I haz the power, 2014-07-19 07:09:25]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sigbjornf@opera.com/401013003/1

[commit-bot: I haz the power, 2014-07-19 07:09:47]

Change committed as 178514

[haraken, 2014-07-19 07:34:45]

LGTM