[net] Better StopCaching() handling for HttpCache::Transaction.

net/http/mock_http_cache.cc

Index: net/http/mock_http_cache.cc
diff --git a/net/http/mock_http_cache.cc b/net/http/mock_http_cache.cc
index d939a3d8f9692f9f6c8eeddeda0e97dd1ccc38ca..adf79f88ac388b1c78e31b140c126071911ff4c2 100644
--- a/net/http/mock_http_cache.cc
+++ b/net/http/mock_http_cache.cc
@@ -495,6 +495,11 @@ void MockDiskCache::GetStats(base::StringPairs* stats) {
 void MockDiskCache::OnExternalCacheHit(const std::string& key) {
 }
 
+void MockDiskCache::ResetCounts() {
+  open_count_ = 0;
+  create_count_ = 0;
+}
+
 void MockDiskCache::ReleaseAll() {
   EntryMap::iterator it = entries_.begin();
   for (; it != entries_.end(); ++it)