RTCRTPStreamStats.ssrc changed type to uint32_t.

As per PR: https://github.com/w3c/webrtc-stats/pull/157

BUG=webrtc:7065, webrtc:7066

Review-Url: https://codereview.webrtc.org/2675583003
Cr-Original-Commit-Position: refs/heads/master@{#16471}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3443bb75a08811a16fd2dd406f72878703f4a861
3 files changed