Oilpan: Build fix after r167288

r167288 broke oilpan's build:
http://chromegw.corp.google.com/i/chromium.webkit/builders/WebKit%252520Linux%252520Oilpan/builds/991/steps/compile/logs/stdio#error1

This CL fixes the build by replacing nullptr with 0.

TBR=tkent

Review URL: https://codereview.chromium.org/170183002

git-svn-id: svn://svn.chromium.org/blink/trunk@167307 bbb929c8-8fbe-4397-9dbb-9b2b20218538