terelius 4311ba59d8 Refactored CL for moving the output to a separate thread.
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}
2016-04-22 19:40:46 +00:00
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