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This records RTC event logs for the duration of each session. This data
is kept in a memory buffer, and limited in size to avoid the buffer
growing indefinitely.

Followup CLs will add the ability to send the recorded log to the
client.

Bug: 1122798
Change-Id: If32a1ea65d93a53ee5cf6e74cd7dbea151fb1214
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380961
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804397}