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 c4cd4c66be5a755979cc387c34f5443636593df3..aea9bc15eac37127709d956b574d845aeac0efcc 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);

[jkarlin, 2017/06/28 15:50:11]

What calls this? I don't see anything in the CL.

+
   RequestPriority priority() const { return priority_; }
   const HttpRequestInfo* request() const { return request_; }
 
@@ -282,6 +286,11 @@ class MockNetworkTransaction
 
   CompletionCallback resume_start_callback_;  // used for pause and restart.
 
+  // Error to be returned from Read. If set it will override all Read logic and
+  // simply return the error code (synchronously/asynchronously based on test
+  // mode).

[Randy Smith (Not in Mondays), 2017/06/27 18:49:38]

This is part of the interface, not the implementation, and so should be in
comments above the "private:" line.   And I think this is moderately big deal (I
tentatively put it in the category of "violation of the principle of least
astonishment", meaning I think it'll take other users of this class by surprise
in more cases than not), so I want to make sure it's called out reasonably
loudly.  Could you make sure that this is included in the doc for SetReadError()
and a reference to it put in the comments before AddMockTransaction()?  That's a
bit unfair, since these classes are very much not well documented currently, but
that makes me more worried about making them confusing to use, not less.

Alternatively: I think this same behavior could be done by specifying a
read_handler in the transaction passed to AddMockTransaction().  Any chance you
could simply use that existing interface and not bother with having to document
new behavior?
 

[shivanisha, 2017/06/28 18:02:33]

Added the error code in MockTransaction and now its definition and handling
corresponds well with the existing return_code member. Renamed the existing
member to start_return_code and this one to read_return_code

+  int read_error_;
+
   base::WeakPtrFactory<MockNetworkTransaction> weak_factory_;
 
 };