Oilpan: Fix issues around key-value GC safety in hash maps

R=ager@chromium.org, zerny@chromium.org
BUG=
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/480313003

git-svn-id: svn://svn.chromium.org/blink/trunk@180951 bbb929c8-8fbe-4397-9dbb-9b2b20218538