Fix the data race of VideoTrackRecorder::Encoder::pause_

|pause_| can be set on the main thread and read on the io thread
at the same time. Declaring |pause_| as an atomic variable
solves the data race problem.

Fuzzer report: https://clusterfuzz.com/testcase?key=5766664522432512

Bug: 988327
Change-Id: Idd43e9d932dc96900b46a09c8b46ba4337505880
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722952
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682194}