Move PlatformChannelPair to //mojo/edk/platform and rename it PlatformPipe.

mojo/edk/system/channel_test_base.cc

Index: mojo/edk/system/channel_test_base.cc
diff --git a/mojo/edk/system/channel_test_base.cc b/mojo/edk/system/channel_test_base.cc
index 67d0a56118e002f770e76d1e185dad9ac15a2cbf..91bb997a9635a86d5f7c2a601cb204b2fe8dcf44 100644
--- a/mojo/edk/system/channel_test_base.cc
+++ b/mojo/edk/system/channel_test_base.cc
@@ -7,9 +7,10 @@
 #include <utility>
 
 #include "base/logging.h"
-#include "mojo/edk/embedder/platform_channel_pair.h"
+#include "mojo/edk/platform/platform_pipe.h"
 #include "mojo/edk/system/raw_channel.h"
 
+using mojo::platform::PlatformPipe;
 using mojo::util::MakeRefCounted;
 
 namespace mojo {
@@ -63,7 +64,7 @@ void ChannelTestBase::ShutdownAndReleaseChannelOnIOThread(unsigned i) {
 void ChannelTestBase::SetUpOnIOThread() {
   CHECK(io_thread()->IsCurrentAndRunning());
 
-  embedder::PlatformChannelPair channel_pair;
+  PlatformPipe channel_pair;
   raw_channels_[0] = RawChannel::Create(channel_pair.handle0.Pass());
   raw_channels_[1] = RawChannel::Create(channel_pair.handle1.Pass());
 }