Fix uninitialized reads in IsDefaultBrowserFirefox

BUG=
TEST=Local DrMemory.
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/19529006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6232 4adac7df-926f-26a2-2b94-8c16560cd09d