Use microsecond timestamp in cricket::VideoFrame.

BUG=webrtc:5740

Committed: https://crrev.com/f30ba114bb33dd1d8643bc640dda2e0c86dbbd32
Cr-Commit-Position: refs/heads/master@{#12348}

Review URL: https://codereview.webrtc.org/1865283002

Cr-Original-Commit-Position: refs/heads/master@{#12358}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: b17712ff894b0bb06c36d12354f0a12def11ee81
11 files changed