Revert 167307 "Oilpan: Build fix after r167288"

Mixing nullptr and 0 causes a build issue
since OwnPtr expects nullptr but RawPtr expects 0.

See https://codereview.chromium.org/170183002/ for more details.


> 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

TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/169683007

git-svn-id: svn://svn.chromium.org/blink/trunk@167310 bbb929c8-8fbe-4397-9dbb-9b2b20218538