Oilpan: Fix DocumentMarkerControllerTest. Need to force GC to get objects to actually disappear.

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

Review URL: https://codereview.chromium.org/222443004

git-svn-id: svn://svn.chromium.org/blink/trunk@170761 bbb929c8-8fbe-4397-9dbb-9b2b20218538