Oilpan: don't do weak processing on null entries in the MatchedPropertiesCache.

BUG=398565
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/428083002

git-svn-id: svn://svn.chromium.org/blink/trunk@179218 bbb929c8-8fbe-4397-9dbb-9b2b20218538