Stop using 'using namespace'.

This CL removes all the instances of 'using namespace' from C++ code
(more info https://abseil.io/tips/153).

Bug: webrtc:9855
Change-Id: Ic940fe87c5047742cfa6d60857d2f97be380ed18
Reviewed-on: https://webrtc-review.googlesource.com/c/113948
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25985}
diff --git a/rtc_base/messagequeue_unittest.cc b/rtc_base/messagequeue_unittest.cc
index d8e8b11..d04d753 100644
--- a/rtc_base/messagequeue_unittest.cc
+++ b/rtc_base/messagequeue_unittest.cc
@@ -23,7 +23,8 @@
 #include "rtc_base/thread.h"
 #include "rtc_base/timeutils.h"
 
-using namespace rtc;
+namespace rtc {
+namespace {
 
 class MessageQueueTest : public testing::Test, public MessageQueue {
  public:
@@ -231,3 +232,6 @@
   t->Post(RTC_FROM_HERE, &handler, 0,
           new ScopedRefMessageData<RefCountedHandler>(inner_handler));
 }
+
+}  // namespace
+}  // namespace rtc