First step to move common accessibility code out of content.

content/common/ax_tree_source.cc

Index: content/common/ax_tree_source.cc
diff --git a/native_client_sdk/src/tests/nacl_io_test/mount_mock.cc b/content/common/ax_tree_source.cc
similarity index 66%
copy from native_client_sdk/src/tests/nacl_io_test/mount_mock.cc
copy to content/common/ax_tree_source.cc
index 1006822ed21767e07ef0032ddefddca842698d98..a3d6a41bbee960ee2827c30619c513f739d3a199 100644
--- a/native_client_sdk/src/tests/nacl_io_test/mount_mock.cc
+++ b/content/common/ax_tree_source.cc
@@ -2,9 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "mount_mock.h"
+#include "content/common/ax_tree_source.h"
 
-MountMock::MountMock() {}
+namespace content {
 
-MountMock::~MountMock() {}
 
+
+}  // namespace content