Reason for revert: breaks downstream code Original issue's description: > Remove unnecessary interface TelephoneEventHandler. > > BUG=webrtc:2795 > > Committed: https://crrev.com/2beb42983ca24e1326a9a7f2c06b3ad740eea2c3 > Cr-Commit-Position: refs/heads/master@{#14346} TBR=henrik.lundin@webrtc.org,solenberg@webrtc.org # Skipping CQ checks because original CL landed less than 1 days ago. NOPRESUBMIT=true NOTREECHECKS=true NOTRY=true BUG=webrtc:2795 Review-Url: https://codereview.webrtc.org/2362673002 Cr-Commit-Position: refs/heads/master@{#14348}