Propagate off-the-recordness to URL request context.

This is necessary because we use `persist:tor' since for hysterical
raisins there's only one normal `private' partition with in_memory_ =
true.  We use the virtual method IsOffTheRecord() to discriminate
instead.

Fixes #608.
Fixes brave/browser-laptop#14392.

Auditors: @darkdh

vendor/brightray/browser/browser_context.cc

@@ -125,7 +125,7 @@ net::URLRequestContextGetter* BrowserContext::CreateRequestContext(
       this,
       static_cast<NetLog*>(BrowserClient::Get()->GetNetLog()),
       GetPath(),
-      in_memory_,
+      IsOffTheRecord(),
       BrowserThread::GetTaskRunnerForThread(BrowserThread::IO),
       protocol_handlers,
       std::move(protocol_interceptors));