Adding method to create process using LowBox token in sandbox code.

sandbox/win/src/sid.h

Index: sandbox/win/src/sid.h
diff --git a/sandbox/win/src/sid.h b/sandbox/win/src/sid.h
index 4656859bec99b62ad05624870069155bf38b7977..442112088a9893d085750a3ceae6bc6afba7cbb2 100644
--- a/sandbox/win/src/sid.h
+++ b/sandbox/win/src/sid.h
@@ -5,6 +5,7 @@
 #ifndef SANDBOX_SRC_SID_H_
 #define SANDBOX_SRC_SID_H_
 
+#include <string>
 #include <windows.h>
 
 namespace sandbox {
@@ -20,6 +21,10 @@ class Sid {
   // Returns sid_.
   const SID *GetPSID() const;
 
+  // Returns sid based on wellknown or provided sid but with unique
+  // subauthority.
+  std::wstring GenerateUniqueSubAuthoritySid();
+
  private:
   BYTE sid_[SECURITY_MAX_SID_SIZE];
 };