Introduce a delegate to avoid hardcoding "chrome-extension" in net/.

chrome/browser/io_thread.h

Index: chrome/browser/io_thread.h
diff --git a/chrome/browser/io_thread.h b/chrome/browser/io_thread.h
index e74f43be1d27e605b61b0326ff7b1fef5f65c28d..e8c220fa470b321dc47398d0cf1438e50f5ac91c 100644
--- a/chrome/browser/io_thread.h
+++ b/chrome/browser/io_thread.h
@@ -47,6 +47,7 @@ class TransportSecurityState;
 class URLRequestContext;
 class URLRequestContextGetter;
 class URLRequestThrottlerManager;
+class URLRequestThrottlerManagerDelegate;
 class URLSecurityManager;
 }  // namespace net
 
@@ -87,6 +88,8 @@ class IOThread : public content::BrowserThreadDelegate {
         proxy_script_fetcher_http_transaction_factory;
     scoped_ptr<net::FtpTransactionFactory>
         proxy_script_fetcher_ftp_transaction_factory;
+    scoped_ptr<net::URLRequestThrottlerManagerDelegate>
+        throttler_manager_delegate;
     scoped_ptr<net::URLRequestThrottlerManager> throttler_manager;
     scoped_ptr<net::URLSecurityManager> url_security_manager;
     // TODO(willchan): Remove proxy script fetcher context since it's not