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

net/http/http_cache_transaction.h

Index: net/http/http_cache_transaction.h
diff --git a/net/http/http_cache_transaction.h b/net/http/http_cache_transaction.h
index dec02f157cba2bad10c2d778991d487872614450..61e5703c35f43efc395cb209f4bbe54c7aab8d55 100644
--- a/net/http/http_cache_transaction.h
+++ b/net/http/http_cache_transaction.h
@@ -179,6 +179,12 @@ class HttpCache::Transaction : public HttpTransaction {
   // Returns the estimate of dynamically allocated memory in bytes.
   size_t EstimateMemoryUsage() const;
 
+  // Sets fail state such that a future Read fails with |error_code|.
+  void SetSharedWritingFailState(int error_code);
+
+  RequestPriority priority() const { return priority_; }
+  PartialData* partial() { return partial_.get(); }
+
  private:
   static const size_t kNumValidationHeaders = 2;
   // Helper struct to pair a header name with its value, for