Adds a new class HttpCache::Writers for multiple cache transactions reading from the network.

net/http/http_transaction_test_util.h

Index: net/http/http_transaction_test_util.h
diff --git a/net/http/http_transaction_test_util.h b/net/http/http_transaction_test_util.h
index 96afead791d093370df60d641e968f2d653e2913..e2839cdb727b6106f58726c7b74410e324ce4d1e 100644
--- a/net/http/http_transaction_test_util.h
+++ b/net/http/http_transaction_test_util.h
@@ -241,6 +241,10 @@ class MockNetworkTransaction
   CreateHelper* websocket_handshake_stream_create_helper() {
     return websocket_handshake_stream_create_helper_;
   }
+
+  // Error to return on a future Read call.
+  void SetReadError(int error);
+
   RequestPriority priority() const { return priority_; }
   const HttpRequestInfo* request() const { return request_; }
 
@@ -282,6 +286,8 @@ class MockNetworkTransaction
 
   CompletionCallback resume_start_callback_;  // used for pause and restart.
 
+  int read_error_;
+
   base::WeakPtrFactory<MockNetworkTransaction> weak_factory_;
 
 };