Oilpan: Fix crashes introduced by style sheet content cache weak processing.

The problem is that null values are put in the stylesheet content cache
(which is bad and is a leak, but we should be able to deal with it).
This mimics the current behavior where you leak if the weak member is
null. I'll see if I can restructure the code to get rid of the leak
in this code for good.

R=erik.corry@gmail.com, oilpan-reviews@chromium.org
BUG=

Review URL: https://codereview.chromium.org/194473004

git-svn-id: svn://svn.chromium.org/blink/trunk@168920 bbb929c8-8fbe-4397-9dbb-9b2b20218538