Implement URLRequestBackoffManager for managing Backoff headers

net/url_request/url_request_context_builder.h

Index: net/url_request/url_request_context_builder.h
diff --git a/net/url_request/url_request_context_builder.h b/net/url_request/url_request_context_builder.h
index a104c4386550a0d43363b6d9f159c48c4866ffb6..f601d8dce1f9f487e09ea266d975f8204f367e44 100644
--- a/net/url_request/url_request_context_builder.h
+++ b/net/url_request/url_request_context_builder.h
@@ -181,6 +181,10 @@ class NET_EXPORT URLRequestContextBuilder {
     throttling_enabled_ = throttling_enabled;
   }
 
+  void set_backoff_enabled(bool backoff_enabled) {
+    backoff_enabled_ = backoff_enabled;
+  }
+
   void SetInterceptors(
       ScopedVector<URLRequestInterceptor> url_request_interceptors);
 
@@ -232,6 +236,7 @@ class NET_EXPORT URLRequestContextBuilder {
 #endif
   bool http_cache_enabled_;
   bool throttling_enabled_;
+  bool backoff_enabled_;
   bool sdch_enabled_;
 
   scoped_refptr<base::SingleThreadTaskRunner> file_task_runner_;