Reland of Adding the ability to create an RtpSender without a track.

(patchset #8 id:140001 of https://codereview.webrtc.org/1413713003/ )

Relanding after fixing CallAndModifyStream to account for new
procedures for adding/removing a track from a stream.

Original issue's description:
> Adding the ability to create an RtpSender without a track.
>
> This CL also changes AddStream to immediately create a sender, rather
> than waiting until the track is seen in SDP. And the PeerConnection now
> builds the list of "send streams" from the list of senders, rather than
> the collection of local media streams.
>
> Committed: https://crrev.com/ac9d92ccbe2b29590c53f702e11dc625820480d5
> Cr-Commit-Position: refs/heads/master@{#10414}

Review URL: https://codereview.webrtc.org/1468113002

Cr-Commit-Position: refs/heads/master@{#10790}

webrtc/base/helpers.h

@@ -39,6 +39,9 @@ bool CreateRandomString(size_t length, std::string* str);
 bool CreateRandomString(size_t length, const std::string& table,
                         std::string* str);
 
+// Generates a (cryptographically) random UUID version 4 string.
+std::string CreateRandomUuid();
+
 // Generates a random id.
 uint32_t CreateRandomId();