Mojo: Move system embedder API files to mojo/system/embedder.

mojo/system/embedder/platform_handle.h

Index: mojo/system/embedder/platform_handle.h
diff --git a/mojo/system/platform_handle.h b/mojo/system/embedder/platform_handle.h
similarity index 86%
rename from mojo/system/platform_handle.h
rename to mojo/system/embedder/platform_handle.h
index ba7f08fb28902e73f1878c65fc76303fff33b08f..d1322911790571e0cd83bdd7f20f0d7b7fb80b36 100644
--- a/mojo/system/platform_handle.h
+++ b/mojo/system/embedder/platform_handle.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef MOJO_SYSTEM_PLATFORM_HANDLE_H_
-#define MOJO_SYSTEM_PLATFORM_HANDLE_H_
+#ifndef MOJO_SYSTEM_EMBEDDER_PLATFORM_HANDLE_H_
+#define MOJO_SYSTEM_EMBEDDER_PLATFORM_HANDLE_H_
 
 #include "build/build_config.h"
 #include "mojo/system/system_impl_export.h"
@@ -44,4 +44,4 @@ struct MOJO_SYSTEM_IMPL_EXPORT PlatformHandle {
 }  // namespace embedder
 }  // namespace mojo
 
-#endif  // MOJO_SYSTEM_PLATFORM_HANDLE_H_
+#endif  // MOJO_SYSTEM_EMBEDDER_PLATFORM_HANDLE_H_