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}
diff --git a/webrtc/base/helpers_unittest.cc b/webrtc/base/helpers_unittest.cc
index 6ea0167..83cc685 100644
--- a/webrtc/base/helpers_unittest.cc
+++ b/webrtc/base/helpers_unittest.cc
@@ -43,16 +43,23 @@
   EXPECT_EQ(256U, random2.size());
 }
 
+TEST_F(RandomTest, TestCreateRandomUuid) {
+  std::string random = CreateRandomUuid();
+  EXPECT_EQ(36U, random.size());
+}
+
 TEST_F(RandomTest, TestCreateRandomForTest) {
   // Make sure we get the output we expect.
   SetRandomTestMode(true);
   EXPECT_EQ(2154761789U, CreateRandomId());
   EXPECT_EQ("h0ISP4S5SJKH/9EY", CreateRandomString(16));
+  EXPECT_EQ("41706e92-cdd3-46d9-a22d-8ff1737ffb11", CreateRandomUuid());
 
   // Reset and make sure we get the same output.
   SetRandomTestMode(true);
   EXPECT_EQ(2154761789U, CreateRandomId());
   EXPECT_EQ("h0ISP4S5SJKH/9EY", CreateRandomString(16));
+  EXPECT_EQ("41706e92-cdd3-46d9-a22d-8ff1737ffb11", CreateRandomUuid());
 
   // Test different character sets.
   SetRandomTestMode(true);