Bug 1382711 - Don't respect a stale pref on Nightly. r=Felipe

This pref is only set and maintained on Release and Beta (via the e10srollout
addon). Respecting it on Beta might cause us to read stale values.

MozReview-Commit-ID: ZDFdM41hw4

--HG--
extra : rebase_source : 2d9e7264b843cc90108c96824f06dc7a228dca28

toolkit/xre/nsAppRunner.cpp

@@ -5190,10 +5190,16 @@ GetMaxWebProcessCount()
     return std::max(1u, optInPrefValue);
   }
 
+#ifdef RELEASE_OR_BETA
+  // For our rollout on Release and Beta, we set this pref from the
+  // e10srollout extension. On Nightly, we don't touch the pref at all,
+  // allowing stale values to disable e10s-multi for certain users.
   if (Preferences::HasUserValue("dom.ipc.processCount.web")) {
     // The user didn't opt in or out so read the .web version of the pref.
     return std::max(1, Preferences::GetInt("dom.ipc.processCount.web", 1));
   }
+#endif
+
   return optInPrefValue;
 }