bug 1361338 - disable atk in content processes r=jld

We proxy all accessibility information for the content through the main process
so its unnecessary and possibly confusing to expose accessibility information
from the content processes.  Further ATK's use of dbus uses syscalls that we
would like to disallow within sandboxed content processes.

ipc/glue/GeckoChildProcessHost.cpp

@@ -746,6 +746,11 @@ GeckoChildProcessHost::PerformAsyncLaunchInternal(std::vector<std::string>& aExt
   if (mProcessType == GeckoProcessType_Content) {
     // disable IM module to avoid sandbox violation
     newEnvVars["GTK_IM_MODULE"] = "gtk-im-context-simple";
+
+    // Disable ATK accessibility code in content processes because it conflicts
+    // with the sandbox, and we proxy that information through the main process
+    // anyway.
+    newEnvVars["NO_AT_BRIDGE"] = "1";
   }
 #endif