Detect notifications only on IPv4 networks.
24 bits of the source IP must be the same as in interface IP.

BUG=295997
TBR=noamsml

Review URL: https://codereview.chromium.org/24325003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224624 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224624