Reduce the scope of rtc::Event::Wait() locking.

Reduces contention on event_mutex_ while taking gettimeofday(). Impact
highly hypothetical at this point, but less locking is better.

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1716563003 .

Cr-Original-Commit-Position: refs/heads/master@{#11706}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7ddc9deb4d7fd6c5396ba55a0ba0e24bf5ef920e