Finish work queue in SctpDataMediaChannelTest.
Finish work queue in SctpDataMediaChannelTest.

Always finishing the work queue prevents memory leak detected in
LeakSanitizer (packet is deleted on the receiver side).

R=jiayl@webrtc.org
BUG=3608,chromium:375154

Review URL: https://webrtc-codereview.appspot.com/28399004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7118e6166924b569805552253aebd8ae6e370bf3

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