Port some fixes in AppRTCDemo.

- Make PeerConnectionClient a singleton.
- Fix crash in CpuMonitor.
- Remove reading constraints from room response.
- Catch and report camera errors.

R=wzh@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43059004

Cr-Commit-Position: refs/heads/master@{#8930}
8 files changed