[cros] Fix DCHECK on startup.
[cros] Fix DCHECK on startup.

Referencing a pref that's not registered on CrOS.


BUG=None
TEST=No DCHECK on first run


Review URL: http://codereview.chromium.org/9721020

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@127489 0039d316-1c4b-4281-b951-d872f2087c98

yitian134/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,adobe/chromium,ropik/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,gavinp/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,adobe/chromium