Null out member so that it will be properly reset in StartLogging()

r=petersont
Review URL: http://codereview.chromium.org/4242

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@2536 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed