Convert rtc_event_log from webrtc::Clock to rtc::TimeMicros.

TBR=pthatcher@webrtc.org
BUG=webrtc:6733

Review-Url: https://codereview.webrtc.org/2515653002
Cr-Original-Commit-Position: refs/heads/master@{#15711}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 306127635e72a31adf351078b17fa0d8ebc479e0
7 files changed