Add //mojo/edk/platform and move platform_task_runners.h there.

mojo/edk/system/slave_connection_manager.cc

Index: mojo/edk/system/slave_connection_manager.cc
diff --git a/mojo/edk/system/slave_connection_manager.cc b/mojo/edk/system/slave_connection_manager.cc
index 6cee86b1d7855b3006fb30505272f454695ec645..0018a60b0167e9938ed8c303eff244c057e21e23 100644
--- a/mojo/edk/system/slave_connection_manager.cc
+++ b/mojo/edk/system/slave_connection_manager.cc
@@ -15,6 +15,7 @@
 #include "mojo/edk/system/message_in_transit.h"
 #include "mojo/edk/util/make_unique.h"
 
+using mojo::platform::TaskRunner;
 using mojo::util::MutexLocker;
 using mojo::util::RefPtr;
 
@@ -46,7 +47,7 @@ SlaveConnectionManager::~SlaveConnectionManager() {
 }
 
 void SlaveConnectionManager::Init(
-    RefPtr<embedder::PlatformTaskRunner>&& delegate_thread_task_runner,
+    RefPtr<TaskRunner>&& delegate_thread_task_runner,
     embedder::SlaveProcessDelegate* slave_process_delegate,
     embedder::ScopedPlatformHandle platform_handle) {
   DCHECK(delegate_thread_task_runner);