Mojo: Move platform handle/channel stuff in system to embedder namespace.

mojo/system/channel.h

Index: mojo/system/channel.h
diff --git a/mojo/system/channel.h b/mojo/system/channel.h
index b52767a55a07abbafeabfe5140707a19ebb70a1a..537564c541d4c3680c57d688f92ca51440aac733 100644
--- a/mojo/system/channel.h
+++ b/mojo/system/channel.h
@@ -68,7 +68,7 @@ class MOJO_SYSTEM_IMPL_EXPORT Channel
   // bidirectional communication channel (e.g., a socket on POSIX, a named pipe
   // on Windows). Returns true on success. On failure, no other methods should
   // be called (including |Shutdown()|).
-  bool Init(ScopedPlatformHandle handle);
+  bool Init(embedder::ScopedPlatformHandle handle);
 
   // This must be called on the creation thread before destruction (which can
   // happen on any thread).