Bug 1508595 - Only set an intercept controller when needed. r=asuth

We only need to expose an intercept controller in SharedWorkers if we're on
the non-parent-intercept version of ServiceWorkers or if e10s is off.
nsDocShell already does this dance and we have to mirror it.

Differential Revision: https://phabricator.services.mozilla.com/D12490

docshell/base/nsDocShell.cpp

@@ -480,6 +480,8 @@ nsDocShell::Create(BrowsingContext* aBrowsingContext)
   // If parent intercept is not enabled then we must forward to
   // the network controller from docshell.  We also enable if we're
   // in the parent process in order to support non-e10s configurations.
+  // Note: This check is duplicated in SharedWorkerInterfaceRequestor's
+  // constructor.
   if (!ServiceWorkerParentInterceptEnabled() || XRE_IsParentProcess()) {
     ds->mInterceptController = new ServiceWorkerInterceptController();
   }