Migrate WebRtcRtpDumpHandler and WebRtcRtpDumpWriter over to sequenced task runner.

BUG=689520

Change-Id: Ib34ee776f40c25b4d4306d54a7bd50756107e13c
Reviewed-on: https://chromium-review.googlesource.com/570998
Commit-Queue: Tommi <tommi@chromium.org>
Reviewed-by: Max Morin <maxmorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487098}
6 files changed