Fix flaky test TestSrtpError, introduced in https://codereview.webrtc.org/1362913004.
Fix flaky test TestSrtpError, introduced in https://codereview.webrtc.org/1362913004.

BUG=webrtc:4690

Review URL: https://codereview.webrtc.org/1380103002

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10137}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4c3a3aea7171f6398deeba71417f6b5df2e819f7

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