When using clang, switch on -Wc++11-narrowing

webrtc/rtc_base/task_queue_libevent.cc

Index: webrtc/rtc_base/task_queue_libevent.cc
diff --git a/webrtc/rtc_base/task_queue_libevent.cc b/webrtc/rtc_base/task_queue_libevent.cc
index b1070da7b7973a714186693f8f4e181d56bb74c5..db267dc80be893aa7715f5dea97ed2a57a93f47c 100644
--- a/webrtc/rtc_base/task_queue_libevent.cc
+++ b/webrtc/rtc_base/task_queue_libevent.cc
@@ -18,6 +18,7 @@
 #include "base/third_party/libevent/event.h"
 #include "webrtc/rtc_base/checks.h"
 #include "webrtc/rtc_base/logging.h"
+#include "webrtc/rtc_base/safe_conversions.h"
 #include "webrtc/rtc_base/task_queue_posix.h"
 #include "webrtc/rtc_base/timeutils.h"
 
@@ -317,7 +318,8 @@ void TaskQueue::PostDelayedTask(std::unique_ptr<QueuedTask> task,
     QueueContext* ctx =
         static_cast<QueueContext*>(pthread_getspecific(GetQueuePtrTls()));
     ctx->pending_timers_.push_back(timer);
-    timeval tv = {milliseconds / 1000, (milliseconds % 1000) * 1000};
+    timeval tv = {rtc::dchecked_cast<int>(milliseconds / 1000),
+                  rtc::dchecked_cast<int>(milliseconds % 1000) * 1000};
     event_add(&timer->ev, &tv);
   } else {
     PostTask(std::unique_ptr<QueuedTask>(