The logging thread is always active. The main thread uses SwapQueues to pass events to the logging thread. The logging thread moves the events to either a RingBuffer history in memory, or to a string which is written to disc.
RtcEventLogImpl constructor takes a clock for easier testing.
BUG=webrtc:4741
Review URL: https://codereview.webrtc.org/1687703002
Cr-Commit-Position: refs/heads/master@{#12476}
