DIAL (Discovery and Launch protocol) extension API skeleton.

This implements the skeleton for a new Chrome extension API for local device discovery. The API will first be restricted to whitelisted extensions only. The API will allow extensions to receive events from a DIAL service running within Chrome which notifies of devices being discovered on the local network.

Spec available here:
https://docs.google.com/a/google.com/document/d/14FI-VKWrsMG7pIy3trgM3ybnKS-o5TULkt8itiBNXlQ/edit

BUG=163288
TBR=ben@chromium.org

Review URL: https://chromiumcodereview.appspot.com/11444020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@172243 0039d316-1c4b-4281-b951-d872f2087c98
scoped_refptr<Extension> LoadManifest(const std::string& dir,
                                      const std::string& test_file) {
  return LoadManifest(dir, test_file, Extension::NO_FLAGS);
}

scoped_refptr<Extension> LoadManifest(const std::string& dir,
                                      const std::string& test_file) {
  return LoadManifest(dir, test_file, Extension::NO_FLAGS);
}
