Reland of Fix flaky EndToEndTest.TransportSeqNumOnAudioAndVideo (patchset #1 id:1 of https://codereview.webrtc.org/2730893002/ )
Reland of Fix flaky EndToEndTest.TransportSeqNumOnAudioAndVideo (patchset #1 id:1 of https://codereview.webrtc.org/2730893002/ )

Reason for revert:
Fixing race and relanding

Original issue's description:
> Revert of Fix flaky EndToEndTest.TransportSeqNumOnAudioAndVideo (patchset #2 id:20001 of https://codereview.webrtc.org/2728103002/ )
>
> Reason for revert:
> Fails on tsan
> http://build.chromium.org/p/client.webrtc/builders/Linux%20Tsan%20v2/builds/10235
>
> Original issue's description:
> > Fix flaky EndToEndTest.TransportSeqNumOnAudioAndVideo
> > Add extra checks to it to simplify diagnostic should it fail again.
> >
> > BUG=webrtc:7292
> >
> > Review-Url: https://codereview.webrtc.org/2728103002
> > Cr-Commit-Position: refs/heads/master@{#16999}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/bcb6004a9d53615a6428e070cbc6d52a2d8e833b
>
> TBR=asapersson@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:7292
>
> Review-Url: https://codereview.webrtc.org/2730893002
> Cr-Commit-Position: refs/heads/master@{#17000}
> Committed: https://chromium.googlesource.com/external/webrtc/+/9a59473f2c403827a3560944c7ffe76d37ab77b9

TBR=asapersson@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7292

Review-Url: https://codereview.webrtc.org/2730673004
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#17006}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4336d73bd3c8df53ec396e9600463c8e162e20b1

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