Oilpan: fix ImageBitmapTest.ImageLiveResourcePriority test.

Hold onto the local HTMLImageElements using persistents so as to keep
them alive across the forced GC that the test runs.

R=haraken@chromium.org,ager@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/296153005

git-svn-id: svn://svn.chromium.org/blink/trunk@174636 bbb929c8-8fbe-4397-9dbb-9b2b20218538