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

net/http/mock_http_cache.h

Index: net/http/mock_http_cache.h
diff --git a/net/http/mock_http_cache.h b/net/http/mock_http_cache.h
index ca0730429f7b401e18987acdf31a3859991b62f0..d568c64db9a5d206a04973c2b82d37d289f97f30 100644
--- a/net/http/mock_http_cache.h
+++ b/net/http/mock_http_cache.h
@@ -144,6 +144,9 @@ class MockDiskCache : public disk_cache::Backend {
   // Returns number of times a cache entry was successfully created.
   int create_count() const { return create_count_; }
 
+  // Returns number of doomed entries.
+  int doomed_count() const { return doomed_count_; }
+
   // Fail any subsequent CreateEntry and OpenEntry.
   void set_fail_requests() { fail_requests_ = true; }
 
@@ -169,6 +172,7 @@ class MockDiskCache : public disk_cache::Backend {
   EntryMap entries_;
   int open_count_;
   int create_count_;
+  int doomed_count_;
   bool fail_requests_;
   bool soft_failures_;
   bool double_create_check_;