Drop nullptr support from oilpan

We noticed that using nullptr in oilpan causes build issues
since the current Blink uses 0 instead of nullptr.
In order not to mix nullptr and 0, this CL drops nullptr from
oilpan.

Review URL: https://codereview.chromium.org/140413008

git-svn-id: svn://svn.chromium.org/blink/trunk@166420 bbb929c8-8fbe-4397-9dbb-9b2b20218538