Add WebRTC logging extension API.

No functionality hooked up in this CL; this is just to get the API itself in place. Will be done in follow-up CL(s).

BUG=TBD

Review URL: https://chromiumcodereview.appspot.com/23885002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225031 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225031