Add Some Comments to the Timeout Checks on Condition Variables

BUG=619037

Review-Url: https://codereview.chromium.org/2341773003
Cr-Commit-Position: refs/heads/master@{#418655}

base/synchronization/condition_variable_posix.cc

@@ -118,6 +118,8 @@ void ConditionVariable::TimedWait(const TimeDelta& max_time) {
 #endif  // OS_ANDROID && HAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC
 #endif  // OS_MACOSX
 
+  // On failure, we only expect the CV to timeout. Any other error value means
+  // that we've unexpectedly woken up.
   DCHECK(rv == 0 || rv == ETIMEDOUT);
 #if DCHECK_IS_ON()
   user_lock_->CheckUnheldAndMark();

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());
   }