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

mojo/system/embedder.h

Index: mojo/system/embedder.h
diff --git a/mojo/system/embedder.h b/mojo/system/embedder.h
index 2431ba2b0279735d6c126d914be777dc5fe9b0b7..12cf2b2b6f3a82588ec24a3d772fe95f7132f1bd 100644
--- a/mojo/system/embedder.h
+++ b/mojo/system/embedder.h
@@ -43,7 +43,7 @@ MOJO_SYSTEM_IMPL_EXPORT void Init();
 struct ChannelInfo;
 typedef base::Callback<void(ChannelInfo*)> DidCreateChannelOnIOThreadCallback;
 MOJO_SYSTEM_IMPL_EXPORT MojoHandle CreateChannel(
-    system::ScopedPlatformHandle platform_handle,
+    ScopedPlatformHandle platform_handle,
     scoped_refptr<base::TaskRunner> io_thread_task_runner,
     DidCreateChannelOnIOThreadCallback callback);