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

net/http/http_transaction_test_util.cc

Index: net/http/http_transaction_test_util.cc
diff --git a/net/http/http_transaction_test_util.cc b/net/http/http_transaction_test_util.cc
index cb46c907ab1793c5825a0bf465ff99e36920147b..a3cc760017accdc29003e3e38133cc2019c080c9 100644
--- a/net/http/http_transaction_test_util.cc
+++ b/net/http/http_transaction_test_util.cc
@@ -237,6 +237,7 @@ MockNetworkTransaction::MockNetworkTransaction(RequestPriority priority,
       sent_bytes_(0),
       socket_log_id_(NetLogSource::kInvalidId),
       done_reading_called_(false),
+      read_error_(0),
       weak_factory_(this) {}
 
 MockNetworkTransaction::~MockNetworkTransaction() {
@@ -305,20 +306,24 @@ int MockNetworkTransaction::Read(net::IOBuffer* buf,
                                  int buf_len,
                                  const CompletionCallback& callback) {
   CHECK(!done_reading_called_);
-  int num = 0;
-  if (read_handler_) {
-    num = (*read_handler_)(content_length_, data_cursor_, buf, buf_len);
-    data_cursor_ += num;
-  } else {
-    int data_len = static_cast<int>(data_.size());
-    num = std::min(static_cast<int64_t>(buf_len), data_len - data_cursor_);
-    if (test_mode_ & TEST_MODE_SLOW_READ)
-      num = std::min(num, 1);
-    if (num) {
-      memcpy(buf->data(), data_.data() + data_cursor_, num);
+  int num = read_error_;
+
+  if (OK == num) {
+    if (read_handler_) {
+      num = (*read_handler_)(content_length_, data_cursor_, buf, buf_len);
       data_cursor_ += num;
+    } else {
+      int data_len = static_cast<int>(data_.size());
+      num = std::min(static_cast<int64_t>(buf_len), data_len - data_cursor_);
+      if (test_mode_ & TEST_MODE_SLOW_READ)
+        num = std::min(num, 1);
+      if (num) {
+        memcpy(buf->data(), data_.data() + data_cursor_, num);
+        data_cursor_ += num;
+      }
     }
   }
+
   if (test_mode_ & TEST_MODE_SYNC_NET_READ)
     return num;
 
@@ -500,6 +505,10 @@ void MockNetworkTransaction::GetConnectionAttempts(
   NOTIMPLEMENTED();
 }
 
+void MockNetworkTransaction::SetReadError(int error) {
+  read_error_ = error;
+}
+
 void MockNetworkTransaction::CallbackLater(const CompletionCallback& callback,
                                            int result) {
   base::ThreadTaskRunnerHandle::Get()->PostTask(