Adding lowbox condition to IsTokenCacheable function.

Problem discovered during PPAPI out of process tests, where it launches multiple processes with different delegates.

BUG=455496
R=rvargas,cpu,jschuh

Review URL: https://codereview.chromium.org/977193002

Cr-Commit-Position: refs/heads/master@{#319194}

sandbox/win/src/broker_services.cc

@@ -88,8 +88,8 @@ bool IsTokenCacheable(const sandbox::PolicyBase* policy) {
   const sandbox::AppContainerAttributes* app_container =
       policy->GetAppContainer();
 
-  // We cannot cache tokens with an app container.
-  if (app_container)
+  // We cannot cache tokens with an app container or lowbox.
+  if (app_container || policy->GetLowBoxSid())
     return false;
 
   return true;