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

mojo/system/platform_channel_pair_posix.cc

Index: mojo/system/platform_channel_pair_posix.cc
diff --git a/mojo/system/platform_channel_pair_posix.cc b/mojo/system/platform_channel_pair_posix.cc
index 8af838f4c3e2028f0ef74604a8ab8c76375b55c0..7b7680b119bbc02429204fbd48ebf124468ea044 100644
--- a/mojo/system/platform_channel_pair_posix.cc
+++ b/mojo/system/platform_channel_pair_posix.cc
@@ -16,7 +16,7 @@
 #include "mojo/system/platform_handle.h"
 
 namespace mojo {
-namespace system {
+namespace embedder {
 
 namespace {
 
@@ -102,5 +102,5 @@ void PlatformChannelPair::ChildProcessLaunched() {
   client_handle_.reset();
 }
 
-}  // namespace system
+}  // namespace embedder
 }  // namespace mojo