Bug 1505067 [wpt PR 13950] - Fix and re-enable simplecall-no-ssrcs.https.html., a...

Bug 1505067 [wpt PR 13950] - Fix and re-enable simplecall-no-ssrcs.https.html., a=testonly

Automatic update from web-platform-testsFix and re-enable simplecall-no-ssrcs.https.html.

With the changes, the test should pass regardless of if the Chrome or
Firefox behavior is used for what to do when no MSIDs are signaled.

Follow-up work is needed to test both the case of "a=msid:-" and no
"a=msid" line whatsoever, awaiting spec issue to be resolved:
https://github.com/w3c/webrtc-pc/issues/2027

// Relying on fippo's approval to land.
TBR=hta@chromium.org

Bug: 901711
Change-Id: Icaf089d206413510aadad54a0c10b2674450385a
Reviewed-on: https://chromium-review.googlesource.com/c/1319612
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605678}

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wpt-commits: bce2269d0f17d2a89247895cbee21530ae83c96e
wpt-pr: 13950