Add back a missing include in metrics_service.cc.
Add back a missing include in metrics_service.cc.

string_util.h is recursively included by one of the other includes in this file,
which is why we don't get a compile error, but it's best to always include what
you use.

BUG=None
TEST=None
Review URL: http://codereview.chromium.org/7065044

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@86612 0039d316-1c4b-4281-b951-d872f2087c98

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