[tor-bugs] #25579 [Applications/Tor Browser]: Bump snowflake/go-webrtc again for trac 21312

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#25579: Bump snowflake/go-webrtc again for trac 21312
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     Reporter:  dcf                       |      Owner:  tbb-team
         Type:  task                      |     Status:  new
     Priority:  Medium                    |  Milestone:
    Component:  Applications/Tor Browser  |    Version:
     Severity:  Normal                    |   Keywords:  snowflake
Actual Points:                            |  Parent ID:
       Points:                            |   Reviewer:
      Sponsor:                            |
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 This is a followup to #25449.

 We discovered additional crashes having to do with the go-webrtc binding
 in #21312. Although we only saw the crashes in the proxy and server
 components, not the client, the attached patch applies the same
 synchronization fixes to the client.

 I did a `testbuild` and put the results here (I'm using it right now to
 file this ticket):
 https://people.torproject.org/~dcf/pt-
 bundle/snowflake/20180321-8.0a4-4a5889af2891/

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25579>
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