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

mojo/edk/system/master_connection_manager.h

Index: mojo/edk/system/master_connection_manager.h
diff --git a/mojo/edk/system/master_connection_manager.h b/mojo/edk/system/master_connection_manager.h
index 33eab674924bd2ce960b230b9d89e63a15f864ce..863b76b90b1b374f743031e78117bcaf1967c53d 100644
--- a/mojo/edk/system/master_connection_manager.h
+++ b/mojo/edk/system/master_connection_manager.h
@@ -10,8 +10,8 @@
 #include <unordered_map>
 
 #include "base/threading/thread.h"
-#include "mojo/edk/embedder/platform_task_runner.h"
 #include "mojo/edk/embedder/scoped_platform_handle.h"
+#include "mojo/edk/platform/task_runner.h"
 #include "mojo/edk/system/connection_manager.h"
 #include "mojo/edk/util/mutex.h"
 #include "mojo/edk/util/ref_ptr.h"
@@ -54,9 +54,8 @@ class MasterConnectionManager final : public ConnectionManager {
   // |delegate_thread_task_runner| should be the task runner for the "delegate
   // thread", on which |master_process_delegate|'s methods will be called. Both
   // must stay alive at least until after |Shutdown()| has been called.
-  void Init(
-      util::RefPtr<embedder::PlatformTaskRunner>&& delegate_thread_task_runner,
-      embedder::MasterProcessDelegate* master_process_delegate)
+  void Init(util::RefPtr<platform::TaskRunner>&& delegate_thread_task_runner,
+            embedder::MasterProcessDelegate* master_process_delegate)
       MOJO_NOT_THREAD_SAFE;
 
   // Adds a slave process and sets up/tracks a connection to that slave (using
@@ -140,7 +139,7 @@ class MasterConnectionManager final : public ConnectionManager {
   // in |Shutdown()| after |private_thread_| is dead. Thus it's safe to "use" on
   // |private_thread_|. (Note that |master_process_delegate_| may only be called
   // from the delegate thread.)
-  util::RefPtr<embedder::PlatformTaskRunner> delegate_thread_task_runner_;
+  util::RefPtr<platform::TaskRunner> delegate_thread_task_runner_;
   embedder::MasterProcessDelegate* master_process_delegate_;
 
   // This is a private I/O thread on which this class does the bulk of its work.