Fix ubsan warnings in BWE tests.

webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc

Index: webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc b/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc
index d67c538a5b1e7f867a53e615137f1115b4375de0..41576516db7409c1fa67f847918b03821a8c199f 100644
--- a/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc
+++ b/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc
@@ -446,13 +446,14 @@ void RemoteBitrateEstimatorTest::CapacityDropTestHelper(
       if (i > 0) {
         bitrate = (kStartBitrate * i + kBitrateDenom / 2) / kBitrateDenom;
       }
-      stream_generator_->AddStream(new testing::RtpStream(
-          kFramerate,                     // Frames per second.
-          bitrate,                        // Bitrate.
-          kDefaultSsrc + i,               // SSRC.
-          90000,                          // RTP frequency.
-          0xFFFFF000 ^ (~0 << (32 - i)),  // Timestamp offset.
-          0));                            // RTCP receive time.
+      uint32_t mask = ~0ull << (32 - i);
+      stream_generator_->AddStream(
+          new testing::RtpStream(kFramerate,          // Frames per second.
+                                 bitrate,             // Bitrate.
+                                 kDefaultSsrc + i,    // SSRC.
+                                 90000,               // RTP frequency.
+                                 0xFFFFF000u ^ mask,  // Timestamp offset.
+                                 0));                 // RTCP receive time.
       bitrate_sum += bitrate;
     }
     ASSERT_EQ(bitrate_sum, kStartBitrate);