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

net/http/http_cache_transaction.cc

Index: net/http/http_cache_transaction.cc
diff --git a/net/http/http_cache_transaction.cc b/net/http/http_cache_transaction.cc
index 94b42d14d7fbd026dbfb14a545a1dff53dd5c8b3..c3670fb7861f314e1603078916d081c3cbe522f5 100644
--- a/net/http/http_cache_transaction.cc
+++ b/net/http/http_cache_transaction.cc
@@ -580,6 +580,11 @@ size_t HttpCache::Transaction::EstimateMemoryUsage() const {
   return 0;
 }
 
+void HttpCache::Transaction::SetSharedWritingFailState(int result) {
+  // TODO(shivanisha): Implement when integrating with HttpCache::Writers.
+  NOTIMPLEMENTED();
+}
+
 //-----------------------------------------------------------------------------
 
 // A few common patterns: (Foo* means Foo -> FooComplete)