Update MacOS implementation DeviceId::GetDeviceId.

Append the MAC address (as string) to the existing DeviceId string
to improve id "uniqueness".

BUG=334348

Review URL: https://codereview.chromium.org/141383002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246562 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246562