PPAPI: Add new PPB_FileRef.MakeDirectory to support exclusive operation

ppapi/tests/test_file_ref.h

Index: ppapi/tests/test_file_ref.h
diff --git a/ppapi/tests/test_file_ref.h b/ppapi/tests/test_file_ref.h
index ff9121562465f33e328edffeac0f0b322d977a19..b574d5403339ee2ff140edcbf37a5a060121cb26 100644
--- a/ppapi/tests/test_file_ref.h
+++ b/ppapi/tests/test_file_ref.h
@@ -34,6 +34,7 @@ class TestFileRef : public TestCase {
   std::string TestGetPath();
   std::string TestGetParent();
   std::string TestMakeDirectory();
+  std::string TestMakeDirectoryExclusive();
   std::string TestQueryAndTouchFile();
   std::string TestDeleteFileAndDirectory();
   std::string TestRenameFileAndDirectory();