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

mojo/edk/test/multiprocess_test_helper.h

Index: mojo/edk/test/multiprocess_test_helper.h
diff --git a/mojo/edk/test/multiprocess_test_helper.h b/mojo/edk/test/multiprocess_test_helper.h
index aa762121b26619fcfe158b6d0054ce14f41f5ff4..152de14f3d6df20b946f1f812f3a80a244760240 100644
--- a/mojo/edk/test/multiprocess_test_helper.h
+++ b/mojo/edk/test/multiprocess_test_helper.h
@@ -16,8 +16,8 @@
 
 namespace mojo {
 
-namespace embedder {
-class PlatformChannelPair;
+namespace platform {
+class PlatformPipe;
 }
 
 namespace test {
@@ -61,7 +61,7 @@ class MultiprocessTestHelper final {
   static platform::ScopedPlatformHandle client_platform_handle;
 
  private:
-  std::unique_ptr<embedder::PlatformChannelPair> platform_channel_pair_;
+  std::unique_ptr<platform::PlatformPipe> platform_channel_pair_;
 
   // Valid after |StartChild()| and before |WaitForChildShutdown()|.
   base::Process test_child_;