Convert some more |base::Lock|s to |mojo::system::Mutex|s.

mojo/edk/system/test_channel_endpoint_client.h

Index: mojo/edk/system/test_channel_endpoint_client.h
diff --git a/mojo/edk/system/test_channel_endpoint_client.h b/mojo/edk/system/test_channel_endpoint_client.h
index c2347d56435076320803dd71831a610d27caab85..bb812f01956e1bef2e498df5057e6a8eca21b7ec 100644
--- a/mojo/edk/system/test_channel_endpoint_client.h
+++ b/mojo/edk/system/test_channel_endpoint_client.h
@@ -7,10 +7,10 @@
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
-#include "base/synchronization/lock.h"
 #include "mojo/edk/system/channel_endpoint.h"
 #include "mojo/edk/system/channel_endpoint_client.h"
 #include "mojo/edk/system/message_in_transit_queue.h"
+#include "mojo/edk/system/mutex.h"
 #include "mojo/public/cpp/system/macros.h"
 
 namespace base {
@@ -49,15 +49,15 @@ class TestChannelEndpointClient final : public ChannelEndpointClient {
  private:
   ~TestChannelEndpointClient() override;
 
-  mutable base::Lock lock_;  // Protects the members below.
+  mutable Mutex mutex_;
 
-  unsigned port_;
-  scoped_refptr<ChannelEndpoint> endpoint_;
+  unsigned port_ MOJO_GUARDED_BY(mutex_);
+  scoped_refptr<ChannelEndpoint> endpoint_ MOJO_GUARDED_BY(mutex_);
 
-  MessageInTransitQueue messages_;
+  MessageInTransitQueue messages_ MOJO_GUARDED_BY(mutex_);
 
   // Event to trigger if we read a message (may be null).
-  base::WaitableEvent* read_event_;
+  base::WaitableEvent* read_event_ MOJO_GUARDED_BY(mutex_);
 
   MOJO_DISALLOW_COPY_AND_ASSIGN(TestChannelEndpointClient);
 };