Oilpan: simplify ImageBitmapTest heap clearing.

For ImageBitmapLiveResourcePriority, use a Persistent to hold onto a
local ImageBitmap object rather than set up and rely on a full stack
scan.

R=wibling@chromium.org,haraken@chromium.org,ager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/292763002

git-svn-id: svn://svn.chromium.org/blink/trunk@174302 bbb929c8-8fbe-4397-9dbb-9b2b20218538