Implement service discovery client for Mac.
chrome.mdns bug fixes.

BUG=280900
R=gene@chromium.org, mfoltz@chromium.org, vitalybuka@chromium.org

Review URL: https://codereview.chromium.org/23484050

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224616 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224616