Bug 1824682 - Add /dev/urandom to rdd process sandbox on Linux r=gcp

On Linuxen without getrandom(), Rust (and Firefox broadly) uses /dev/urandom
as a fallback. Rust uses it for a few things, notably hashmaps... and will
panic if it can't use it.

Differential Revision: https://phabricator.services.mozilla.com/D174336

security/sandbox/linux/broker/SandboxBrokerPolicyFactory.cpp

@@ -820,6 +820,7 @@ SandboxBrokerPolicyFactory::GetRDDPolicy(int aPid) {
 
   AddSharedMemoryPaths(policy.get(), aPid);
 
+  policy->AddPath(rdonly, "/dev/urandom");
   // FIXME (bug 1662321): we should fix nsSystemInfo so that every
   // child process doesn't need to re-read these files to get the info
   // the parent process already has.