Disable RampUpTest.UpDownUpTransportSequenceNumberRtx on Mac.

Due to flakiness. See bug.

TBR=stefan@webrtc.org

Bug: webrtc:8878
Change-Id: Iece487ad9883fbe6209d25291fb70bb52c8afbd3
Reviewed-on: https://webrtc-review.googlesource.com/54601
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22059}
diff --git a/call/rampup_tests.cc b/call/rampup_tests.cc
index 3a85214..6b20ecd 100644
--- a/call/rampup_tests.cc
+++ b/call/rampup_tests.cc
@@ -578,7 +578,15 @@
   RunBaseTest(&test);
 }
 
-TEST_F(RampUpTest, UpDownUpTransportSequenceNumberRtx) {
+// TODO(bugs.webrtc.org/8878)
+#if defined(WEBRTC_MAC)
+#define MAYBE_UpDownUpTransportSequenceNumberRtx \
+  DISABLED_UpDownUpTransportSequenceNumberRtx
+#else
+#define MAYBE_UpDownUpTransportSequenceNumberRtx \
+  UpDownUpTransportSequenceNumberRtx
+#endif
+TEST_F(RampUpTest, MAYBE_UpDownUpTransportSequenceNumberRtx) {
   std::vector<int> loss_rates = {0, 0, 0, 0};
   RampUpDownUpTester test(3, 0, 0, kStartBitrateBps,
                           RtpExtension::kTransportSequenceNumberUri, true,