Bug 1540839 - Assert on browsing context creation in chrome; r=nika

We shouldn't be creating browsing contexts in chrome. Add
assertion (behind pref that guards the rest of this patch set) to make
sure that doesn't happen.

Differential Revision: https://phabricator.services.mozilla.com/D30561

docshell/base/BrowsingContext.h

@@ -348,6 +348,7 @@ class BrowsingContext : public nsWrapperCache,
 
   // Create an IPCInitializer object for this BrowsingContext.
   IPCInitializer GetIPCInitializer() {
+    MOZ_ASSERT(IsContent());
     IPCInitializer init;
     init.mId = Id();
     init.mParentId = mParent ? mParent->Id() : 0;