Enabling IPv6 socket recv timestamp test, and making less flaky.

The test worked by sleeping a certain time, then checking that the
difference between recv timestamps before and after the sleep was
within some margin of the requested sleep time.

However, this means that imprecision of SleepMs makes the test flaky.
This source of flakiness can be removed by comparing to the actual
time slept instead of the requested time.

Also making the margin larger, to further reduce the likelihood of
flakiness.

R=pthatcher@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/2111043004 .

Cr-Commit-Position: refs/heads/master@{#13733}
diff --git a/webrtc/base/socket.h b/webrtc/base/socket.h
index 7db9459..73dfdbb 100644
--- a/webrtc/base/socket.h
+++ b/webrtc/base/socket.h
@@ -151,6 +151,7 @@
   virtual int Connect(const SocketAddress& addr) = 0;
   virtual int Send(const void *pv, size_t cb) = 0;
   virtual int SendTo(const void *pv, size_t cb, const SocketAddress& addr) = 0;
+  // |timestamp| is in units of microseconds.
   virtual int Recv(void* pv, size_t cb, int64_t* timestamp) = 0;
   virtual int RecvFrom(void* pv,
                        size_t cb,