Fix uninits introduced by r241374.

Make sure quick_check_enabled_ is initialized in ProxyService and
ProxyService::InitProxyResolver constructors.

BUG=329339
TBR=ellyjones@chromium.org,szym@chromium.org

Review URL: https://codereview.chromium.org/146833004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246944 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246944