Start documenting the threading model

webrtc/modules/pacing/paced_sender_unittest.cc

Index: webrtc/modules/pacing/paced_sender_unittest.cc
diff --git a/webrtc/modules/pacing/paced_sender_unittest.cc b/webrtc/modules/pacing/paced_sender_unittest.cc
index 6c5e3df2809733d967f5b6124ad3b4802ab4ae81..7a1833edf4783f2d3b66cce7a4208479bc960d51 100644
--- a/webrtc/modules/pacing/paced_sender_unittest.cc
+++ b/webrtc/modules/pacing/paced_sender_unittest.cc
@@ -634,7 +634,9 @@ TEST_F(PacedSenderTest, Pause) {
 
   for (int i = 0; i < 10; ++i) {
     clock_.AdvanceTimeMilliseconds(5);
-    EXPECT_EQ(0, send_bucket_->TimeUntilNextProcess());
+    // TimeUntilNextProcess must not return 0 when paused.  If it does,
+    // we risk running a busy loop, so ideally it should return a large value.
+    EXPECT_LT(1000, send_bucket_->TimeUntilNextProcess());
     send_bucket_->Process();
   }