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

mojo/edk/system/master_connection_manager.cc

Index: mojo/edk/system/master_connection_manager.cc
diff --git a/mojo/edk/system/master_connection_manager.cc b/mojo/edk/system/master_connection_manager.cc
index f4db9134da2f0fe26b1c25feca52a22e7291e3d5..4e78897a6c59f723de6bba091ea917b915ef4ca6 100644
--- a/mojo/edk/system/master_connection_manager.cc
+++ b/mojo/edk/system/master_connection_manager.cc
@@ -25,6 +25,7 @@
 #include "mojo/edk/util/waitable_event.h"
 #include "mojo/public/cpp/system/macros.h"
 
+using mojo::platform::TaskRunner;
 using mojo::util::AutoResetWaitableEvent;
 using mojo::util::MutexLocker;
 using mojo::util::RefPtr;
@@ -342,7 +343,7 @@ MasterConnectionManager::~MasterConnectionManager() {
 }
 
 void MasterConnectionManager::Init(
-    RefPtr<embedder::PlatformTaskRunner>&& delegate_thread_task_runner,
+    RefPtr<TaskRunner>&& delegate_thread_task_runner,
     embedder::MasterProcessDelegate* master_process_delegate) {
   DCHECK(delegate_thread_task_runner);
   DCHECK(master_process_delegate);