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 51c0db70256cbd31505ca20ffba6486ab4c36aad..95b1ebf401607a8514021d08ec186bc91c43d16b 100644
--- a/net/http/http_cache_transaction.h
+++ b/net/http/http_cache_transaction.h
@@ -167,6 +167,12 @@ class HttpCache::Transaction : public HttpTransaction {
   // Returns the estimate of dynamically allocated memory in bytes.
   size_t EstimateMemoryUsage() const;
 
+  // Any subsequent Read or IO callback should fail with the given result.
+  void SetSharedWritingFailState(int result);
+
+  RequestPriority priority() { 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