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 545e05c8c82522e703c215b748d0dd1a00e364bb..c753c6da7044638ddecfcfc0ed35b4af29864d28 100644
--- a/net/http/http_cache_transaction.h
+++ b/net/http/http_cache_transaction.h
@@ -183,6 +183,12 @@ 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