Fix -Wimplicit-fallthrough build after https://chromium-review.googlesource.com/891224.

Bug: 177475
Change-Id: I878036841f95d7bdadb199f759f13bcdf887b505
Reviewed-on: https://chromium-review.googlesource.com/895804
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533372}

content/renderer/media/rtc_peer_connection_handler.cc

@@ -825,6 +825,7 @@ class PeerConnectionUMAObserver : public webrtc::UMAObserver {
             "WebRTC.PeerConnection.KeyProtocol",
             static_cast<webrtc::KeyExchangeProtocolType>(counter),
             webrtc::kEnumCounterKeyProtocolMax);
+        break;
       default:
         // The default clause is expected to be reached when new enum types are
         // added.