Add Some Comments to the Timeout Checks on Condition Variables

base/synchronization/condition_variable_win.cc

Index: base/synchronization/condition_variable_win.cc
diff --git a/base/synchronization/condition_variable_win.cc b/base/synchronization/condition_variable_win.cc
index d5f24e4438753471dc38b0e48a0ae680477eafb2..8bece6eb34dbfc965ccee65b08051c54ad9fff79 100644
--- a/base/synchronization/condition_variable_win.cc
+++ b/base/synchronization/condition_variable_win.cc
@@ -35,6 +35,11 @@ void ConditionVariable::TimedWait(const TimeDelta& max_time) {
 #endif
 
   if (!SleepConditionVariableSRW(&cv_, srwlock_, timeout, 0)) {
+    // On failure, we only expect the CV to timeout. Any other error value means
+    // that we've unexpectedly woken up.
+    // Note that WAIT_TIMEOUT != ERROR_TIMEOUT. WAIT_TIMEOUT is used with the
+    // WaitFor* family of functions as a direct return value. ERROR_TIMEOUT is
+    // used with GetLastError().
     DCHECK_EQ(static_cast<DWORD>(ERROR_TIMEOUT), GetLastError());
   }